Generic Git Provider: validations: check that remote exists

We need to validate early that the remote provided by the customer exists.

Given we have no access to a Provider API to check if the remote repository exists, we can perform a good approximation of this using [`ls-remote`](https://git-scm.com/docs/git-ls-remote.html).

The library we use for performing git actions, go-git, supports [ls-remote](https://github.com/go-git/go-git/blob/master/_examples/ls-remote/main.go) calls.

We should perform this validation at the time we instantiate the git client for a generic git provider, either in the FluxAddonClient (https://github.com/aws/eks-anywhere/blob/main/pkg/addonmanager/addonclients/fluxaddonclient.go#L91) or in the GitFactory.

Dependent on https://github.com/aws/eks-anywhere/pull/1934