Added dynamic_map_metadata service for requesting the map metadata.

[hiddewie]

Issue #45.

Adds a service for the map metadata: /dynamic_map_metadata. Requires adding a separate map metadata service, because it does not exist in nav_msgs.

I've added a new test for this service, just like for the normal /dynamic_map service. All tests pass.

[hiddewie]

@k-okada Thanks for the comment. :+1: I Rebased the commits.