[backend] a working /imagen endpoint

[ofey404]

Part of #3 .

This PR switch /imagen's API backend to stable.

Also, it did a slight refactor to the configuration loading mechanism.

It can still load from .env / environment variable, but in a structured way.

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Roadmap:

• First of all, we need a config object/file, in order to tweak things on testing.
• Set up and run /imagen, to verify it's in a working state.

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Those are pushed to following PRs:

• Write the test, based on the working endpoint.
• Create a mock for external moving parts like hugging face model.
• Then refactor after we have proper tests.

[ofey404]

How to integrate pytest into bazel:

pytest support #240 | rules/python