Add keychain option to notarization_info

[MScottBlake]

I ran into an issue where notarization did not work without also specifying the keychain. During troubleshooting, I noticed this output from the notarytool store-credentials command:

Credentials saved to Keychain.
To use them, specify `--keychain-profile "notarization_credentials" --keychain /path/to/custom_keychain.keychain-db`

When using a custom keychain (not login or system keychain), the credentials may not be available to xcrun notarytool submit without specifying the keychain location.

This change adds keychain as an optional key within notarization_info if keychain_profile is configured. It should not break any backwards compatibility.