Add support for filepath-based authn.Keychain

[hasheddan]

Currently, it is possible to configure the location of the auth config file in the default keychain, but only if there is not already a config file at the default path. Furthermore, the name of the file (config.json) is hard-coded and cannot be overridden. It would be helpful to support passing an arbitrary path to credentials to be used, perhaps as part of a multi-keychain. This would be straightforward to implement in consume in the same manner as other alternative keychain implementations, such as github and k8schain.

[imjasonh]

This seems valuable. There are at least a couple ways we could do it, and I'm curious which one fits your needs best:

1. document an env var that authn.DefaultKeychain will look for before considering ${DOCKER_CONFIG} (e.g., GGCR_DOCKER_CONFIG) which specifies the full path of the file. For example, GGCR_DOCKER_CONFIG=test/cfg.json crane ls ....
2. implement authn.NewConfigKeychain(filename string) that returns the otherwise normal DefaultKeychain implementation, but pointed at the full filename instead of ${DOCKER_CONFIG}/config.json, for example:

remote.Image(ref,
    remote.WithAuthFromKeychain(authn.NewConfigKeychain("test/cfg.json"))

@jonjohnsonjr might have even better ideas.

[hasheddan]

@imjasonh I think both could be implemented as independent functionality, but I was thinking more in the vein of (2) as it allows the natural keychain precedence to be used with potentially multiple config files.