registry: Accept wildcard is not supported

RFC 9110 allows wildcard (e.g. */*, application/*) to be specified for the Accept header.

However, as of commit 97b1d649c4938d0f608d96454d6a8326b1f96acd, the parsing of the Accept header implemented by the registry only supports explicit content types.

It would be useful to follow the relevant web standard here and allow wildcard Accept headers so a content-agnostic client can avoid enumerating all possible manifest types, a practice which is both bulky and potentially fragile to maintain over time.

For example, passing the header:

Accept: */*

to a manifest GET request should allow all possible manifest types.