Move logic for explicit routing headers to google-api-core

[parthea]

See feedback in https://github.com/googleapis/gapic-generator-python/pull/2131/files#r1751065571 from review of PR https://github.com/googleapis/gapic-generator-python/pull/2131 which calls out that the logic for explicit routing headers which exists in _client_macros.j2 could be moved to google-api-core, similar to how we have gapic_v1.routing_header.to_grpc_metadata

This logic: https://github.com/googleapis/gapic-generator-python/blob/4c5de8791795f8101f6ec66f80b8a8e5e9a21822/gapic/templates/%25namespace/%25name_%25version/%25sub/services/%25service/_client_macros.j2#L150-L164

Could be refactored into a helper function in google-api-core similar to gapic_v1.routing_header.to_grpc_metadata

https://github.com/googleapis/gapic-generator-python/blob/4c5de8791795f8101f6ec66f80b8a8e5e9a21822/gapic/templates/%25namespace/%25name_%25version/%25sub/services/%25service/_client_macros.j2#L167-L170