Implement configuration resolution

[ubaskota]

Issue #, if available:

Description of changes:
This PR implements the following:

• Config source interface and environment variable source
• Config resolver that resolves config values from sources
• Config property descriptor along with a custom resolver and validators
• Updates codegen files that generate config.py to support config resolution for newly added config variables:
  • retry_strategy, region and sdk_ua_app_id (in progress)
• Unit tests covering all newly implemented features

Tests

• Manual tests verifying config resolution for the added config variables
• End-to-end tests from the generated client confirming expected resolution behavior
• All existing tests pass

Note: There are 2 minor PRs yet to be merged.

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