chore: add generator version to config

[JoeWang1127]

In this PR:

• Add generator version to generation config.

The library generation job failed due to config doesn't have a generator version, see https://github.com/googleapis/java-bigtable/actions/runs/10712723965/job/29703584291.

[JoeWang1127]

Generation workflow failed. This is expected because config in main branch (baseline config) doesn't have a generator version.

  File "/src/cli/entry_point.py", line 129, in generate
    baseline_config=from_yaml(baseline_generation_config_path),
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/library_generation/model/generation_config.py", line 170, in from_yaml
    parsed_config = GenerationConfig(
                    ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/library_generation/model/generation_config.py", line 49, in __init__
    self.gapic_generator_version = GenerationConfig.__set_generator_version(
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/library_generation/model/generation_config.py", line 92, in __set_generator_version
    raise ValueError(
ValueError: Environment variable GENERATOR_VERSION is not set when the generator version is not specified in the generation config.