Cpp metadata encoder

[Hailios]

Pull Request

Add metadata encoder to C++. It is currently next to the decoder class. It does not use any persistent state so does not really need to be instantiated.

Effort was made to stick close to the Rust reference implementation. Which at the time included having the string size bug which was later fixed in Rust in this commit https://github.com/databento/dbn/commit/aede39b73c9858eb95ce2fd678ef0378ec38bc9e this PR does not initially contain the corresponding update.

Type of change

Please delete options that are not relevant.

• [X] New feature (non-breaking change which adds functionality)
• [ ] This change requires a documentation update

How has this change been tested?

1 unit test.

and

wrote metadata to a file and read it back from python, seems fine bar potential lack of DBN understanding from the humans doing the testing. seems fine.