tools: add perplexity AI

[sklinkert]

This PR enables AI Agents to use perplexity AI in order to retrieve data from the web.

I'm not affiliated with Perplexity.

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.

[FluffyKebab]

@sklinkert Looking good. Just one more thing before this can go in: All exported symbols should be commented.

[FluffyKebab]

@sklinkert + Handle tool callbacks in Call

[sklinkert]

@FluffyKebab Thanks for reviewing it! Let me know if there is anything else I can do

[FluffyKebab]

LGTM!