Add manifest merging support

[jedevc]

• [x] Requires https://github.com/docker/cli/pull/3990.

- What I did

Introduced support for to merge manifest lists and OCI indexes to the docker manifest command.

- How I did it

We refactor the manifest store to save and load groups of manifests, instead of only one. On disk, these manifests take the form of suffixed indexes, e.g. "_2", "_3", etc (note that we ignore "_1", which is permitted to allow a backwards-compatible change with the previous disk format).

Then, only the commands need small updates - "create" now looks up multiple manifests and saves multiple manifests, while "annotate" now annotates all the matching manifests.

- How to verify it

Use docker manifest with multiple docker manifests - e.g. docker manifest create library/ubuntu library/alpine

- Description for the changelog

Add support for merging manifest lists to docker manifest

- A picture of a cute animal (not mandatory but encouraged)

cc @thaJeztah @crazy-max

[thaJeztah]

This one probably can be rebased now (as the other PR was merged)

[codecov-commenter]

Codecov Report

Merging #3994 (577dc07) into master (3ae101f) will decrease coverage by 0.06%. The diff coverage is 63.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3994      +/-   ##
==========================================
- Coverage   59.15%   59.09%   -0.06%     
==========================================
  Files         287      287              
  Lines       24723    24763      +40     
==========================================
+ Hits        14624    14634      +10     
- Misses       9214     9236      +22     
- Partials      885      893       +8