Add scalar.proto, generate stubs, implement converter, and add unit tests.

[mjohanse-emr]

What does this Pull Request accomplish?

The changes made in this PR will live in the nipanel package temporarily to allow for development and testing of gRPC conversion for Scalar with Units types. I will explain more about the long term location for these changes at the end of this section.

• Copy the ScalarData message from ASW into this python package
• Generate the grpc stubs for the new .proto file
• Create a Converter that converts scalar objects from python <--> protobuf.
• Write unit tests to exercise this converter class.
• I had to add a dependency on the nitypes package because it contains the Scalar python class.

In the long term...

• The scalar.proto file will move into the ni-apis package
• The generated grpc stubs will move into the ni-apis-python repo/package
• The conversion code will move into the ni-apis-python repo/package
• The dependency from nipanel on nitypes can be removed

Why should this Pull Request be merged?

Fulfills temporary development/testing defined by AB#3157159.

What testing has been done?

Unit tests pass.

[github-actions]

Test Results

 10 files  ±  0   10 suites  ±0   13s ⏱️ +3s
 78 tests + 11   78 ✅ + 11  0 💤 ±0  0 ❌ ±0 
750 runs  +110  750 ✅ +110  0 💤 ±0  0 ❌ ±0 

Results for commit 6eca4ac. ± Comparison against base commit 541273f.

♻️ This comment has been updated with latest results.