llms: allow specifying JSON Schema

[AkihiroSuda]

This commit adds llms.WithJSONSchema(schema *jsonschema.Definition) so as to enforce structured outputs: https://ollama.com/blog/structured-outputs

This fixes an issue that an LLM produces JSON with an unexpected schema.

Supported LLMs: ollama, openai

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).
• [X] 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.
• (Not applicable) References existing implementations as appropriate.
• [X] Contains test coverage for new functions.
• [X] Passes all golangci-lint checks.

[alegrey91]

Any plan to merge this PR? I would be interested too.

[AkihiroSuda]

Rebased to solve the merge conflict

[AkihiroSuda]

@tmc Could you take a look?

[AkihiroSuda]

@tmc May I ask if you have an interest in this, or is this not planned to be merged?

[tmc]

Quite interested and I want to see it in. I've been developing a supplemental project that has kept me away from maintenance tasks here but I will look this evening.

[tmc]

I have https://github.com/tmc/langchaingo/tree/prep-v0.1.15 pushed which has structured output changes that improve provider support but I like what I see here, need to get to bed but will consider refactoring this PR to fit on top of those commits (or you can do so yourself). Thanks for your contirbution!