OCI Support for manifests command and server tags page

[elerch]

This PR allows OCI images to be displayed through the manifest command. It also fixes my scenario on #173 which was caused by the create date being determined via a docker v1 manifest. That is now determined by getting a v2/OCI manifest, then acquiring the resultant config blob from the registry that should have the create date (we luck out that both docker v2 and OCI config json have createdDate in the base object).

[athos-ribeiro]

Thank you, I am quite interested in this feature as well!

What if the endpoint returns an OCI index instead of an image manifest?

[elerch]

The endpoint shouldn't return an index. The accept header I send to the server is only asking for v2 and OCI manifest types.

That said, if it were to return an index for some reason, there's no longer validation of the return data, so reg manifest would return the resultant data, and the tags page rendered from the server command would fail to operate.

[athos-ribeiro]

Hi @jessfraz,

@elerch branch has been diverging a bit from upstream, and his changes seem to be quite useful!

Do you need help with a thorough review on thins (or any other) PR and issues here? I'd be more than willing to help providing those and pinging you when they are done :)