Simplify Go Endpoints + Add Tests (#120)

This MR represents a major refactor of the Go codebase, as well as introducing tests for the handlers. The MR also introduces an endpoint to shutdown or restart the Go server, which may be useful for clients who want to refresh the state of the plugin after checking out branches. Finally, this MR adds a contributing document for users who want to make feature changes.

README.md

@@ -29,6 +29,7 @@ https://github.com/harrisoncramer/gitlab.nvim/assets/32515581/dc5c07de-4ae6-4335
   - [MR Approvals](#mr-approvals)
   - [Pipelines](#pipelines)
   - [Reviewers and Assignees](#reviewers-and-assignees)
+  - [Restarting or Shutting down](#restarting-or-shutting-down)
 - [Keybindings](#keybindings)
 - [Troubleshooting](#troubleshooting)
 
@@ -324,6 +325,27 @@ require("dressing").setup({
 })
 ```
 
+### Restarting or Shutting Down
+
+The `gitlab.nvim` server will shut down automatically when you exit Neovim. However, if you would like to manage this yourself (for instance, restart the server when you check out a new branch) you may do so via the `restart` command, or `shutdown` commands, which
+both accept callbacks.
+
+```lua
+require("gitlab.server").restart()
+```
+
+For instance you could set up the following keybinding to close and reopen the reviewer when checking out a new branch:
+
+```lua
+local gitlab = require("gitlab")
+vim.keymap.set("n", "glB", function ()
+    require("gitlab.server").restart(function () 
+        vim.cmd.tabclose()
+        gitlab.review() -- Reopen the reviewer after the server restarts
+    end)
+end)
+```
+
 ## Keybindings
 
 The plugin does not set up any keybindings outside of the special buffers it creates,
@@ -333,6 +355,7 @@ as `gl` does not have a special meaning in normal mode):
 
 ```lua
 local gitlab = require("gitlab")
+local gitlab_server = require("gitlab.server")
 vim.keymap.set("n", "glr", gitlab.review)
 vim.keymap.set("n", "gls", gitlab.summary)
 vim.keymap.set("n", "glA", gitlab.approve)