support settings through didChangeConfiguration notification

[aspeddro]

This PR add support to enable/disable settings through didChangeConfiguration notification.

Settings

• CodeLens
• Extended Hover

Example using Neovim

require('lspconfig').ocamllsp.setup {
  cmd = {'ocamllsp'},
  on_attach = on_attach,
  settings = {
    extendedHover = { 
      enable = true 
    },
    codelens = {
      enable = false
    }
  },
  capabilities = capabilities,
}

Close #1102

[coveralls]

Pull Request Test Coverage Report for Build 3748

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 104 (1.92%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 16.818%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ocaml-lsp-server/src/ocaml_lsp_server.ml	1	6	16.67%
ocaml-lsp-server/src/configuration.ml	1	14	7.14%
ocaml-lsp-server/src/config_data.ml	0	84	0.0%

Files with Coverage Reduction	New Missed Lines	%
ocaml-lsp-server/src/configuration.ml	4	18.18%

Totals
Change from base Build 3733:	-0.07%
Covered Lines:	3934
Relevant Lines:	23391

---

💛 - Coveralls

[3Rafal]

Please add a changelog note here:
https://github.com/ocaml/ocaml-lsp/blob/master/CHANGES.md

[3Rafal]

We have tests for:
hover
hover extended
code lens

Can we add tests that change default configuration and check if it is applied?

[3Rafal]

Thanks @aspeddro for this PR. It looks good, but I think it would be nice to add suggested improvements. Feel free to ask me any questions in case something is not clear. :)

[aspeddro]

Related to this PR:

We can remove the ocamllsp/hoverExtended request introduced in #561. This is now supported by most clients and is within specification.