Use to_jsonfunctions generated server side with grapqhql_ppx

[ylecornec]

Relevant PR: #11072

[ylecornec]

Relevant PRs: #11072~ for develop, #11158 for compatible

This approach seems to work as tested in issue #10951.

• test the wrapper with ocaml-graphql-server 0.13.0 (that should be updated soon). See issue test the graphql_wrapper library with a recent version of ocaml-graphql-server #11267.
• document the wrapper
• modify graphql_ppx to make using custom encoders optional
• pass tests on the develop branch
• adapt changes to the compatible branch
• adapt changes to the most recent version of graphql_ppx (updated in Use upstream graphql_ppx, upgrade ppxlib #11114): This branch of the graphql_ppx fork can now make use of custom encoders for query arguments (which are defined alongside the server (see this commit)).
• open PRs to develop/compatible with only the changes of branch ylecornec/graphql_experiments relevant to this issue.
• pull upstream changes in graphql_ppx fork to support OCaml 4.14