Use `to_json`functions generated server side with grapqhql_ppx

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

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

[x] test the wrapper with ocaml-graphql-server 0.13.0 (that should be updated soon). See issue #11267.
[x] document the wrapper
[x] modify graphql_ppx to make using custom encoders optional
[x] pass tests on the develop branch
[x] adapt changes to the compatible branch
[x] adapt changes to the most recent version of graphql_ppx (updated in #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)).
[x] open PRs to develop/compatible with only the changes of branch ylecornec/graphql_experiments relevant to this issue.
[x] pull upstream changes in graphql_ppx fork to support OCaml 4.14

MinaProtocol / mina