service: Revise DiscoveryClient API

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

ServiceInfo:

• Add display_name field

DiscoveryClient:

• Add a register_service method:
  • Not measurement-specific
  • Accepts the full ServiceLocation, not only the insecure port
  • Returns registration id instead of using a member variable
• Deprecate register_measurement_service
• Deprecate properties that should not be public

GrpcService:

• Call new register_service method
• Save registration ID

tests:

• Update test_discovery_client.py to test the new & old APIs
• Update test_discovery_client.py to use a mock DiscoveryServiceStub

Why should this Pull Request be merged?

Make the public API more general and follow the proto file more closely.

Hide implementation details.

What testing has been done?

Ran pytest.

[github-actions]

Test Results

     12 files  ±  0       12 suites  ±0   2m 0s ⏱️ +5s
   198 tests +  4     169 ✔️ +  4    29 💤 ±0  0 ❌ ±0 
2 364 runs  +48  2 016 ✔️ +48  348 💤 ±0  0 ❌ ±0 

Results for commit 1ed046b. ± Comparison against base commit 90d6064.

♻️ This comment has been updated with latest results.