change(fastapi): rework /rpc (get)

This reworks the base implementation of the RPC to use a
class called RPC for handling of requests. Took a bit of
a different approach than PHP in terms of exposed methods,
but it does end up achieving the same goal, with one additional
error: "Request type '{type}' is not yet implemented."

For FastAPI development, we'll stick with:

- If the supplied 'type' argument has an alias mapping in
  RPC.ALIASES, we convert the type argument over to its alias
  before doing anything. Example: 'info' is aliased to 'multiinfo',
  so when a user requests type=info, it is converted to type=multiinfo.
- If the type does not exist in RPC.EXPOSED_TYPES, the following
  error is produced: "No request type/data specified."
- If the type **does** exist in RPC.EXPOSED_TYPES, but does not
  have an implemented `RPC._handle_{type}_type` function, the
  following error is produced: "Request type '{type}' is not yet
  implemented."

Signed-off-by: Kevin Morris <kevr@0cost.org>
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