Add context cache as a request level cache (#22294)

To avoid duplicated load of the same data in an HTTP request, we can set
a context cache to do that. i.e. Some pages may load a user from a
database with the same id in different areas on the same page. But the
code is hidden in two different deep logic. How should we share the
user? As a result of this PR, now if both entry functions accept
`context.Context` as the first parameter and we just need to refactor
`GetUserByID` to reuse the user from the context cache. Then it will not
be loaded twice on an HTTP request.

But of course, sometimes we would like to reload an object from the
database, that's why `RemoveContextData` is also exposed.

The core context cache is here. It defines a new context
```go
type cacheContext struct {
	ctx  context.Context
	data map[any]map[any]any
        lock sync.RWMutex
}

var cacheContextKey = struct{}{}

func WithCacheContext(ctx context.Context) context.Context {
	return context.WithValue(ctx, cacheContextKey, &cacheContext{
		ctx:  ctx,
		data: make(map[any]map[any]any),
	})
}
```

Then you can use the below 4 methods to read/write/del the data within
the same context.

```go
func GetContextData(ctx context.Context, tp, key any) any
func SetContextData(ctx context.Context, tp, key, value any)
func RemoveContextData(ctx context.Context, tp, key any)
func GetWithContextCache[T any](ctx context.Context, cacheGroupKey string, cacheTargetID any, f func() (T, error)) (T, error)
```

Then let's take a look at how `system.GetString` implement it.

```go
func GetSetting(ctx context.Context, key string) (string, error) {
	return cache.GetWithContextCache(ctx, contextCacheKey, key, func() (string, error) {
		return cache.GetString(genSettingCacheKey(key), func() (string, error) {
			res, err := GetSettingNoCache(ctx, key)
			if err != nil {
				return "", err
			}
			return res.SettingValue, nil
		})
	})
}
```

First, it will check if context data include the setting object with the
key. If not, it will query from the global cache which may be memory or
a Redis cache. If not, it will get the object from the database. In the
end, if the object gets from the global cache or database, it will be
set into the context cache.

An object stored in the context cache will only be destroyed after the
context disappeared.

modules/cache/context.go

@@ -0,0 +1,92 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cache
+
+import (
+	"context"
+	"sync"
+
+	"code.gitea.io/gitea/modules/log"
+)
+
+// cacheContext is a context that can be used to cache data in a request level context
+// This is useful for caching data that is expensive to calculate and is likely to be
+// used multiple times in a request.
+type cacheContext struct {
+	ctx  context.Context
+	data map[any]map[any]any
+	lock sync.RWMutex
+}
+
+func (cc *cacheContext) Get(tp, key any) any {
+	cc.lock.RLock()
+	defer cc.lock.RUnlock()
+	if cc.data[tp] == nil {
+		return nil
+	}
+	return cc.data[tp][key]
+}
+
+func (cc *cacheContext) Put(tp, key, value any) {
+	cc.lock.Lock()
+	defer cc.lock.Unlock()
+	if cc.data[tp] == nil {
+		cc.data[tp] = make(map[any]any)
+	}
+	cc.data[tp][key] = value
+}
+
+func (cc *cacheContext) Delete(tp, key any) {
+	cc.lock.Lock()
+	defer cc.lock.Unlock()
+	if cc.data[tp] == nil {
+		return
+	}
+	delete(cc.data[tp], key)
+}
+
+var cacheContextKey = struct{}{}
+
+func WithCacheContext(ctx context.Context) context.Context {
+	return context.WithValue(ctx, cacheContextKey, &cacheContext{
+		ctx:  ctx,
+		data: make(map[any]map[any]any),
+	})
+}
+
+func GetContextData(ctx context.Context, tp, key any) any {
+	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		return c.Get(tp, key)
+	}
+	log.Warn("cannot get cache context when getting data: %v", ctx)
+	return nil
+}
+
+func SetContextData(ctx context.Context, tp, key, value any) {
+	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		c.Put(tp, key, value)
+		return
+	}
+	log.Warn("cannot get cache context when setting data: %v", ctx)
+}
+
+func RemoveContextData(ctx context.Context, tp, key any) {
+	if c, ok := ctx.Value(cacheContextKey).(*cacheContext); ok {
+		c.Delete(tp, key)
+	}
+}
+
+// GetWithContextCache returns the cache value of the given key in the given context.
+func GetWithContextCache[T any](ctx context.Context, cacheGroupKey string, cacheTargetID any, f func() (T, error)) (T, error) {
+	v := GetContextData(ctx, cacheGroupKey, cacheTargetID)
+	if vv, ok := v.(T); ok {
+		return vv, nil
+	}
+	t, err := f()
+	if err != nil {
+		return t, err
+	}
+	SetContextData(ctx, cacheGroupKey, cacheTargetID, t)
+	return t, nil
+}