Hot swappable API

[amiller]

Right now, the API base URL is specified in teleport.env, with the API key passed in by the host (private.env).

This is because the choice of RPC providers does need to be constrained as part of the TCB. But, it's a little rigid because we can only have one.

Instead, what we should do is have a list of allowed RPC providers in teleport.env. Then the actual RPC+key can be provided in private.env and the enclave proceeds as long as the whole url starts with one of the prefixes in the allowlist.