Add proto and tests for NI-DMM

[DavidCurtiss]

• [x] This contribution adheres to CONTRIBUTING.md.

~~[ ] I've updated CHANGELOG.md if applicable.~~

• [x] I've added tests applicable for this pull request

What does this Pull Request accomplish?

Adds nidmm.proto, and system tests for it.

List issues fixed by this Pull Request below, if any.

None

What testing has been done?

Ran system tests on a machine with the required prerequisites installed. Also ran on my dev machine, where ni_grpc_device_server is not installed (but other NI software is), to see that it skips the tests.