Entryway update handle flow, pds and agent caching

[devinivy]

A few misc fixes, improvements, and tweaks to the entryway setup:

• Fix getAccountByEmail() which was giving a null for the user's did.
• Cache getPds(did) results which is in the hot path of proxying.
• Cache AtpAgents for the entryway's pdses.
• Update entryway flow for updating account handles: should use pds's admin.updateAccountHandle endpoint to avoid entryway and pds identity.updateHandle routes depending on each other.
• Only include didDoc alongside credentials when the pds endpoint is one of the entryway's pdses.

[dholms]

looks great 💪