Replace maybenot submodule with direct ffi re-export wrapper crate

[faern]

This PR removes maybenot as a submodule. And instead adds a local simple wrapper (re-export?) crate named maybenot-ffi. All this crate does is to depend on the real maybenot-ffi crate and re-exports the entire API of it.

This change allows us to depend on maybenot-ffi from crates.io instead of having it as a git dependency (submodule). This in turn allows us to to track maybenot-ffi and all its dependencies in the parent repository and workspace lockfile, simplifying dependency management and verification.

This force us to commit the maybenot.h header to this repository. This is explained in the readme. It should no longer be automagically generated. This removes the build time dependency on cbindgen and helps with reproducible builds also.

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This change is 

[faern]

Reviewable status: 0 of 9 files reviewed, all discussions resolved (waiting on @MarkusPettersson98)

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maybenot-ffi/Cargo.toml line 3 at r1 (raw file):

[package]
name = "maybenot-ffi"
edition = "2021"

This crate is part of our upstream cargo workspace. But we still can't and should not inherit package fields and linting rules here. We can't do that, since this fork of wireguard-go should be buildable without being part of a parent workspace, so this crate can't assume it is. Secondly, many of the package fields are not relevant/correct, such as repository url. Linting etc is not interesting since there is no code in this library really.

[MarkusPettersson98]

Reviewed 8 of 9 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @faern)