fix: use lambda pattern for bedrock config env vars

[skamenan7]

What does this PR do?

Improved bedrock provider config to read from environment variables like AWS_ACCESS_KEY_ID. Updated all
fields to use default_factory with lambda patterns like the nvidia provider does.

Now the environment variables work as documented.

Closes #3305

Test Plan

Ran the new bedrock config tests:

python -m pytest tests/unit/providers/inference/bedrock/test_config.py -v

Verified existing provider tests still work:
python -m pytest tests/unit/providers/test_configs.py -v