Support Bring Your Own Key (BYOK) for agents

[KieranBond]

The problem(s)

1. If you do not have a paid plan, running out of tokens is fatal and renders the app considerably less useful.
2. Some companies will not be happy with their code and product data going through PostHog. They'll want to leverage existing agreements they have with providers.

Proposed solution

• Add support for going direct to model providers
  1. Add support for providing API keys for supported model providers.
  2. Add support for connecting to model providers through OAuth.