Introduce registry package

[darkowlzz]

internal/registry package consolidates all the registry provider logins.
The internal/registry/login package contains a login Manager which
manages logins for all the providers. For testability, it provides methods to
modify the provider client configurations.
internal/registry/{aws/azure/gcp} packages contain clients for logging
into the respective registry. The client APIs are mostly similar across all the
providers, except for the small details related to overriding certain
configurations for testing purposes. Each of the providers have test
coverage to solidify the expected behavior in different scenarios.

This depends on #275 for testing to ensure that the native registry login
tests work before and after these changes.

NOTE: I'd like to gain some confidence in this package, maybe an IRC
release with this package, before moving it to fluxcd/pkg repo. It'd be
easier to fix any potential issues if it's in the same repo with all the
integration tests.

Part of #264

[darkowlzz]

@pjbgf thanks for reviewing the actual login code. Since we didn't had any automated tests before for the cloud providers, I tried to keep the existing login code as they were and wrap them in a new structure, just to ensure I don't break anything. Now that we have automated tests, I'll go through your comments and address them based on actual testing.

[darkowlzz]

I've made some changes based on the review comments and tested it using the test automation.

[stefanprodan]

LGTM

Thanks @darkowlzz 🏅

[pjbgf]

fantastik @darkowlzz! 🔝🔝

LGTM

[hiddeco]

LGTM. Merci @darkowlzz 🙇