Reduce the volume of HEAD requests by caching proxy cache ManifestExist calls

[mxm-tr]

Is your feature request related to a problem? Please describe.
When pulling many artifacts at the same time on a container proxy-cache, we can still trigger the rate limiting on the upstream registries and get 429 Too Many Requests errors.

This is in part caused by HEAD requests being sent for each artifact pull.

Describe the solution you'd like

This feature request aims to reduce the load on upstream registries by caching head requests when pulling from a proxy-cache registry.

The solution could consist of a cache for calls to HeadManifest, perhaps configurable to be valid for a certain period of time?

Describe the main design/architecture of your solution

1. Use a new cache key in the proxy controller cache, such as:

cache:manifestexists:<repo>:<ref>

2. Define its lifetime to a value that would prevent rate limiting from being triggered (10s?) in the proxy-controller

manifestExistsCacheInterval = 10 * time.Second

3. Before running remote.ManifestExist, run a cache fetch on the proxy controller cache.

If the cache is invalid or the key is not found, run remote.ManifestExist, and save a boolean in the proxy controller cache.

Describe the development plan you've considered

Additional context
Since this feature would change the default behavior of the proxy-cache, it may trigger more discussions.

[mxm-tr]

Proposal was drafted here: goharbor/community#261