service: Clean up grpc_servicer.py

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

grpc_servicer.py:

• Remove unnecessary content from docstrings. This class is internal, so we don't need to document each attribute and parameter.
• Delete low-value, "play-by-play" comments
• Delete random capitalization
• Use kwargs to initialize protobuf fields when convenient to do so
• Single-source _frame_metadata_dict()
• Prefix member variables with underscore to indicate that they are private
• Remove unnecessary type hints for member variables (the type checker can infer these)

Why should this Pull Request be merged?

Make grpc_servicer.py easier to read and edit.

What testing has been done?

Ran poetry run pytest -v

[github-actions]

Test Results

     12 files  ±0       12 suites  ±0   2m 48s ⏱️ +27s
   215 tests ±0     186 ✔️ ±0    29 💤 ±0  0 ❌ ±0 
2 568 runs  ±0  2 220 ✔️ ±0  348 💤 ±0  0 ❌ ±0 

Results for commit 7ddbddc. ± Comparison against base commit aa17b56.