Make grpclib truly optional

[AdrienVannson]

Fix #84

[Copilot]

Pull Request Overview

This PR makes grpclib truly optional by restructuring imports and introducing dependency-specific test fixtures. It addresses scenarios where grpclib may not be available in the environment while still allowing the core betterproto2 functionality to work.

Key changes:

• Moves grpclib-specific imports into a conditional module structure
• Adds pytest fixtures to skip tests when optional dependencies are not available
• Updates test generation to use different client generation modes based on grpclib availability

Reviewed Changes

Copilot reviewed 31 out of 34 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
betterproto2/src/betterproto2/grpclib/init.py	New module providing grpclib imports in a conditional structure
betterproto2/src/betterproto2/init.py	Removes direct grpclib import from main module
betterproto2_compiler/src/betterproto2_compiler/templates/*.j2	Updates templates to use qualified grpclib imports
betterproto2_compiler/tests/*.py	Adds client_generation parameter support
betterproto2/tests/*.py	Adds dependency check fixtures and updates test imports
.github/workflows/ci.yml	Adds CI job to test without optional dependencies

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