Add keyring integration for git credentials

[jtfmumm]

This PR adds native keyring integration for persisting and fetching git credentials. On successful authentication for a git repo, credentials are persisted to the system keyring if keyring-provider = "native". Future invocations will be able to find these credentials even in the absence of a git credential helper (though you must currently still provide a username on future invocations, see #10866).

Credentials are stored in the system keyring with a uv-credentials: prefix (see #14559), e.g., the keyring service for a GitHub repo would be named uv-credentials:https://github.com/<org>/<repo>.

TODO:

• Add a test for uv sync (there is currently only a new test for uv add). I've successfully tested uv sync locally but ideally we should have an automated test.

Depends on #14559.

[jtfmumm]

This method was switched to async for calling KeyringProvider::store_if_native, as was store_credentials_from_url below.

[jtfmumm]

This method was switched to async in order to call KeyringProvider::fetch_if_native. As a result, a immediately invoked closure below was converted to a spawn_blocking call. It's worth noting that this change holds whether or not the keyring provider is configured to be native.