Support sharing keychain with kSecAttrAccessGroup

[markst]

Checklist

• I have read the relevant SDK documentation and have not found a suitable solution.
• I agree to the terms within the SAP Code of Conduct.

Describe the problem you'd like to have solved.

Apple keychains can share access to keychain items among a collection of apps:

https://developer.apple.com/documentation/security/keychain_services/keychain_items/sharing_access_to_keychain_items_among_a_collection_of_apps/

We'd like to provide this feature across our apps.

Describe the ideal solution.

Providing the flexibility to specify the shared keychain group or injecting our own keychain storage variant.

Appears currently the KeychainStorageFactory is a concrete internal type which provides the SecItemCopyMatching search query.

Alternatives or workarounds

Alternative is to persist our own shared keychain group storing the token & secret in which we provide to GigyaCore on launch.