meson: wire up credential helpers

meson: wire up credential helpers

We've got a couple of credential helpers in "contrib/credential", all
of which aren't yet wired up via Meson. Do so.

Note that ideally, we'd also wire up t0303 to be executed with each of
the credential helpers to verify their functionality. Unfortunately
though, none of them pass the test suite right now, so this is left for
a future change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>