Add perplexity completion example

[sklinkert]

This PR adds an example for using the perplexity API. The API is OpenAI client-compatible.

• Perplexity API documentation
• Supported Models by perplexity

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.
• [x] References existing implementations as appropriate.
• [x] Contains test coverage for new functions.
• [x] Passes all golangci-lint checks.

[sklinkert]

@tmc can you please merge this? pretty straight-forward PR :)

let me know if something is missing!