docs: add bleRPC to third-party RPC implementations

[tdaira]

What

Adds bleRPC to docs/third_party.md under RPC Implementations.

This page invites projects to "send us a pull request to update this page," so here is one.

About the project

bleRPC (https://blerpc.net) is an open-source RPC framework that carries Protocol Buffers messages over Bluetooth Low Energy (GATT). It uses:

• nanopb for on-device encode/decode (nRF / Zephyr firmware),
• generated clients for Python, Kotlin, Swift, Dart, and TypeScript (from .proto),
• a small container/command framing layer for BLE MTUs, with optional end-to-end encryption.

It's Apache-2.0 licensed. Added to the active list; not affiliated with or endorsed by Google, per the page's disclaimer.

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