callbacks: add standard interface and logger

[FluffyKebab]

This adds an interface for callbacks and incorporates it into tools, the agent executor, the vector store retriever, all LLMs and the LLM chain. Incorporating callbacks into all chains is yet do be done.

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.

[tmc]

I think we should propagate ctx in these calls, thoughts?

[tmc]

Great work, comment on the interface.

[tmc]

I went ahead and rebased and threaded ctx through.