Add on_initialized hook

[m-lima]

Addresses a common issue with large workspaces that take longer to load and cause errors such as:
rust-lang/rust-analyzer#10910
https://www.reddit.com/r/neovim/comments/qwlheb/delay_vimlspcodelensrefresh_until_rustanalyzer_is/

This works by allowing passing a callback to be run once serverStatus with quiescent is sent (similar to inlayHints)

An example configuration:

tools = {
  on_initialized = function()
    vim.cmd([[
      augroup SuperDuperAus
        autocmd CursorHold                      *.rs silent! lua vim.lsp.buf.document_highlight()
        autocmd CursorMoved,InsertEnter         *.rs silent! lua vim.lsp.buf.clear_references()
        autocmd BufEnter,CursorHold,InsertLeave *.rs silent! lua vim.lsp.codelens.refresh()
        autocmd BufWritePre                     *.rs silent! lua vim.lsp.buf.formatting_sync()
      augroup END
    ]])
  end,
}

[simrat39]

Could you please add this to the readme as well?