GcpCompute.TokenProvider that returns a fixed token.

Useful for tests, the Compute emulator, or environments where a token is injected out-of-band. The arg may be:

• a binary — used directly as the bearer token;

• a %{token: ...} map — returned as-is;

• a zero-arity function — invoked to produce {:ok, token} / {:error, _}.

  GcpCompute.Config.local(project: "my-project", token: "ya29.fake")

  => token_provider: {GcpCompute.TokenProvider.Static, "ya29.fake"}

Anything else returns {:error, %GcpCompute.Error{reason: :invalid_token_provider_arg}}. The error :message names only the type of the rejected arg, never its value — a mis-shaped arg is usually a credential (%{"access_token" => ...}, {:bearer, ...}), and :message ends up in logs and crash reports. The raw value is kept in :body, which GcpCompute.Error's Inspect redacts.