OCI: Add flatpak_oci_registry_get_token() method

This tries to do a HEAD request to the repository and if that fails
with 401 uses the OCI mechanism to (given a basic auth token) generate
a token for the download.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f