* internal/registry source: return error if requested provider version protocols are not supported
* getproviders: move responsibility for protocol compatibility checks into the registry client
The original implementation had the providercache checking the provider
metadata for protocol compatibility, but this is only relevant for the
registry source so it made more sense to move the logic into
getproviders.
This also addresses an issue where we were pulling the metadata for
every provider version until we found one that was supported. I've
extended the registry client to unmarshal the protocols in
`ProviderVersions` so we can filter through that list, instead of
pulling each version's metadata.
* internal/providercache: verify that the provider protocol version is
compatible
The public registry includes a list of supported provider protocol
versions for each provider version. This change adds verification of
support and adds a specific error message pointing users to the closest
matching version.
Kristin Laemmert