Storage helper refresh token support

[hickford]

So far the following storage helpers support the necessary attributes for OAuth token refresh:

Helper	password_expiry_utc	oauth_refresh_token
cache	Git 2.40	Git 2.41
store	❌	❌
wincred	Git 2.41	Git 2.44
libsecret	Git 2.43	Git 2.43
osxkeychain	Git 2.45	Git 2.45

You can test any helper using Git test suite t0303-credential-external.sh, eg. GIT_TEST_CREDENTIAL_HELPER=osxkeychain ./t0303-credential-external.sh.