llms: add googleai llm option `WithHTTPClient`

[skrishnan-sap]

Changes

• Adds an option WithHTTPClient to Google AI LLM implementation, which passes a consumer provided http.Client to the underlying vendor maintained client. The new option is used also by the vertex llm implementation.
• The new option can be used for troubleshooting (example using httputil.DebugHTTPClient) or providing flexibility to consume LLM services behind proxies, set additional headers etc.
• Similar options are already available on the OpenAI LLM options as well as the existing PaLM legacy LLM

Issue: fixes https://github.com/tmc/langchaingo/issues/838

PR Checklist

• [x] Read the Contributing documentation.
• [x] Read the Code of conduct documentation.
• [x] 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.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• [x] Describes the source of new concepts.
• [x] References existing implementations as appropriate.
• [x] Contains test coverage for new functions.
• [x] Passes all golangci-lint checks.

[skrishnan-sap]

841 provides similar functionality - waiting to check whether author would like to take over unit tests.

[skrishnan-sap]

Closing in favour of #841 .