Add support for registry v3 JWK thumbprint key ID format

[evanebb]

Fixes #386.
This is a fairly quick and dirty fix, feel free to burn this PR down :)

As mentioned in that issue, the v3 version of the registry no longer supports libtrust key IDs. There are multiple alternative options to choose from, but the simplest one to implement for this project is using the JWK thumbprint of the public key as the key ID instead.

For every certificate present in the rootcertbundle passed to the registry, it'll add the public key to the trusted keys identified by the JWK thumbprint: https://github.com/distribution/distribution/blob/63d3892315c817c931b88779399a8e9142899a8e/registry/auth/token/accesscontroller.go#L346-L348
So, if you pass this JWK thumbprint in the key ID header in the token, the registry can select the proper signing key using this thumbprint.

This PR allows configuring this through a new directive in the configuration file, namely token.disable_legacy_key_id.
If set to true, it will pass the JWK thumbprint in the key ID header instead of the libtrust key ID. It defaults to false for now, to avoid accidental breakage when updating setups using the v2 registry.