Bring cohttp source code into tezos/tezos but only as data.

What

PART 4 - #7349 (closed)

Brings cohttp into the monorepo (the v5.3.1 release version).

This MR DOES NOT bring the entire history of cohttp into tezos/tezos. The reasoning behind this is that we do not need it, as we have not evolved our code according to changes from this project. So, we prefer to avoid polluting the commit history.

Why

Similar to data-encoding, resto and prometheus, we want to have cohttp into the monorepo so developers can make MRs across the library boundary and avoid the release process. Another reason is because we want to integrate some fixes meant to stop the memory leak found in the %Non-Blocking RPCs and proposed in %Improve and consolidate RPC UX.

By bringing it into tezos, we encourage tezos developers to pick up the maintenance and improvement of the project.

The MR only brings the code but doesn't do any of the integration so that it can be merged more easily. This is because the rebase process is complex.

How

cohttp/ is marked as data only in dune and is excluded from build file generation in manifest/main.ml

Manually testing the MR

This MR should have no impact apart from the directory cohttp/ being present. All current uses of the repository (make, other builds, tests, etc.) should work as usual.