Add reverse_proxy to Networking and sap to REST and API

[slogsdon]

Adds two Plug-related libraries:

• reverse_proxy under Networking, alphabetically between reagent and sise. A Plug-based reverse proxy for routing, header rewrites, and basic load balancing.
• sap under REST and API, alphabetically between plug_rest and signaturex. A Plug toolkit for modeling HTTP request handling as a decision tree of composable combinators.

Both are MIT licensed and published on Hex (reverse_proxy, sap).

Format follows the README convention (* [name](link) - description.), entries placed alphabetically, no duplicate links.

[slogsdon]

Heads-up on the second commit: the CI Check README.md job was failing because of an unrelated pre-existing line, * [ReqLLM](https://github.com/agentjido/req_llm) - LLM Client supporting over 100+ LLM Providers and Models, which is missing the trailing period that tests/check.exs requires. That same failure has been blocking master CI since October. Added a one-character fix in a separate commit so this PR can go green; happy to drop it if you'd rather land it via a different PR.