Feature Request: Client TLS Certificate Support in fetch()

Would be beneficial for auth to services that require it, or for authenticating to "another backend" without custom auth wiring.
Current workaround is falling back to `node:https` but the native TLS client seems quite close to handling it

I suppose it'd have to either be a non-standard flag to `fetch`, be configured project-level (impacting the global TLS_CONNECTOR), or an entirely new `convex:https` library

Start with fetch() and the native TLS client, then compare the current node:https workaround and the global TLS_CONNECTOR mentioned in the issue. Done would mean client TLS certificate authentication has a defined configuration or API path without custom auth wiring.

Written by the indexing model from the issue text.