[evals] ERD / design out server credential storage and model restrictions (custom provider)

[mikeldking]

As a user I want to be able to store a credential (or set of credentials) to phoenix securely (symetric hash) so that people on my team can re-use those credentials. These credentials should be set by admins but can be used by members (not usable by viewers).

Design out a system where the credential(s) are stored and capture:

• the sdk to use (openai, bedrock)
• a restriction on models?
• any custom headers
• certs(?)

[mikeldking]

on @mikeldking whether this is something only admins can do