An Elixir atproto OAuth and XRPC client

Make PLC configurable #33

Merged jola.dev
targeting main
Merged opened by and assigned to jola.dev
targeting main

This is useful for local dev where you might have a local PDS and PLC running. Latch would still try to look up the account in https://plc.directory, even for a local PDS, and fail because it doesn't exist.

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