Ref #4057
In this issue we discussed the release issue of OCaml binding. One solution is the current PR.
In this PR, the version of opendal is fixed so that the core directory is not required when building, so that when releasing, we can package this binding code separately.
At the same time, this PR also tested the results of building different OCaml versions and modified the OCaml version restrictions.
After this PR is merged, release OCaml binding requires the following steps:

1. During release, package bindings/ocaml separately into something similar to apache-opendal-$release_version-binding-ocaml-src.tar.gz, and create its sha512 hash.
2. Copy the opendal.opam file in binding/ocaml and append the following code

url {
   src: "https://github.com/apache/opendal/path_to_ocaml_binding_code.tar.gz"
   checksum: [
"sha512=hash"
   ]
}

3. Go to https://github.com/ocaml/opam-repository to submit a PR. The content is the opendal.opam file modified in the previous step.
4. Wait for the PR to be merged after the ci of opam is passed.