feat(lsp): allow user defined per lsp on_attach additions

[mehalter]

This allows additions to the on_attach function for specific language servers in the lsp.server-settings table. This was completely incorrectly handled and honestly could be considered just a flat out bug until now. For example, disabling formatting for a single language server can now look like this:

return {
  lsp = {
    ["server-settings"] = {
      html = {
        on_attach = function(client, bufnr)
          client.resolved_capabilities.document_formatting = false
        end
      }
    }
  }
}

Also makes it easier to disable our custom on_attach for specific languages, like if a user wants to use tsserver formatting they can just do this:

return {
  lsp = {
    ["server-settings"] = {
      tsserver = {
        on_attach = function() end
      }
    }
  }
}