Support using the environment variable GEMINI_API_KEY in addition to GOOGLE_API_KEY

[WilliamHua]

promptlayer.run requires GOOGLE_API_KEY to be set when using a Google model.

Add in support to also allow users to alternatively use GEMINI_API_KEY (official google docs here use GEMINI_API_KEY as the environment variable).

We already use GOOGLE_API_KEY elsewhere in our code base so we cannot set this environment variable.

[WilliamHua]

Should I bump the version? Not super familiar with python libraries

[Jped]

Yeah bump the versions here:

• https://github.com/WilliamHua/prompt-layer-library/blob/1da049ad45bae8a901c61aed6e1afdd9fad153b0/pyproject.toml#L3
• https://github.com/WilliamHua/prompt-layer-library/blob/1da049ad45bae8a901c61aed6e1afdd9fad153b0/promptlayer/__init__.py#L3