Implement a `set_mode` service in `drone_control`

Depends #7 and #6 

This should create a way for us to set the mode in drone_control using enums like GUIDED, LAND, etc etc.

Remember there should be a `common_srvs` entry, if not just create a srv directory in this package for now (you'll refactor later)

There should be a function for setting the mode and also a callback that calls that function using the service proxy you create.