The test suite should include a demo application written in C11

[pavel-kirienko]

We should provide some sort of example written entirely in c that we compile and run as a test just to be sure we're actually compatible with C and not getting away with stuff because we use the C++ compiler for all our tests.

_Originally posted by @thirtytwobits in https://github.com/OpenCyphal-Garage/libudpard/pull/34#discussion_r1251350157_

The application is to serve both as a test and as a usage example. The work on it should be commenced after the RX pipelines are implemented. Perhaps we need an e2e test against PyCyphal as well.

[pavel-kirienko]

• Part of the test suite is written in C.
• A separate demo written in C will soon be provided in the demos repository.