feat(nvim): add HTTP client plugin

I went to GitHub to find a HTTP client plugin that I could use to
replace Postman, and found https://github.com/rest-nvim/rest.nvim.

It seems to work well. Thanks @NTBBloodbath!

roles/neovim/files/lua/opdavies/plugins.lua

@@ -121,6 +121,9 @@ return packer.startup(function()
   use "kristijanhusak/vim-dadbod-completion"
   use "kristijanhusak/vim-dadbod-ui"
 
+  -- HTTP client
+  use "rest-nvim/rest.nvim"
+
   -- Text objects
   use "kana/vim-textobj-indent"
   use "kana/vim-textobj-user"