googleai: fix user provided embedding model getting overridden by default model

[ancalabrese]

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

This PR fixes an issue with the googleai client where the default embedding models were overriding the user provided embedding model.

• Added options in the internal PaLM client to configure the models to use
• Map the user provided options to the internal ones
• Added a new WithGRPCConn option in the external googleai client:
  • This is needed to pass a mock grpc client to be able to test that the embedding request is using the user provided client

Fixes #1276