Cache tokens in memory

[AlexGustafsson]

Processing an image can require multipel calls to an OCI registry, especially if there are many pages worth of tags to retrieve. Currently, each such request results in one uncached request to get a token for the OCI registry. We should try to cache these in memory to reduce the number of requests made to registries.

[AlexGustafsson]

GHCR doesn't report the lifetime of tokens, making this quite difficult. Docker Hub uses a JWT, but relying on the token's format might not be a good idea in the long run.

[AlexGustafsson]

Registries SHOULD use Www-Auth, so perhaps just try once and then, if it doesn't work, use the appropriate authenticator with a token. If it fails, update the token? There are some standard warnings and errors for things like a token being expired, but it's unclear how many registries actually implement them.