Add a new gRPC service to request southbound schema information

See merge request danet/api!23

For the prompt mode in the CLI we need to transport the southbound
specific ygot generated yang schema.

Allow requesting Plugins via API

See merge request danet/api!21

Refactor current .proto definitions

Closes #10

See merge request danet/api!22

updated api proto to be able to receive missing change uuid on change requests and send sbi type on creation

Moved did back from SetPathListRequest to ChangeRequest

repeated tag was lost in refactoring, now reintroduced

This reverts commit b509bb7b.

Created new RPCs for each 'oneof'.