[perf] imgpkg seems to make lots of repeated requests to `/v2` endpoint

# Description

running `imgpkg pull --debug` I notice lots of requests being made to the registry `/v2` endpoint, trying to get a token.

When pulling a bundle with lots of images / nested bundles, I think this may contribute to a performance degradation.

It would be nice to know:
- how much performance we lose by making repeated calls to `/v2` and assuming the registry requires a `bearer token`, repeated calls to an oauth server
- assuming it is causing excessive slowness, can we somehow cache the bearer token avoiding the need to make these repeated calls.

Reference:
- https://github.com/google/go-containerregistry/issues/740