Implement allowlist admin api

[Mirko-von-Leipzig]

Summary

Adds an optional administrator REST API to the sequencer which supports:

1. inserting a new invitation code to the allowlist
2. adding an account ID directly to the allowlist (no registration required)
3. querying the state of an account or invitation code

Changelog

[[entry]]
scope       = "node"
impact      = "added"
description = "Added an admin REST API to the sequencer for managing the account allowlist"

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[SantiagoPittella]

Not a very deep review, but overall looked good. The endpoints are going to be limited at the proxy/LB level, right?