feat: added whomai cmd to coder cli (#13814)

* feat: added whomai cmd to coder cli
* refactor: update Coder CLI's whoami command to use client URL instead of deployment config
* feat(cli): add unit tests for the whoami command
* chore(docs): add coder command to fetch authenticated user info
* chore(doc): update help desc

cli/testdata/coder_--help.golden

@@ -55,6 +55,7 @@ SUBCOMMANDS:
                       date
     users             Manage users
     version           Show coder version
+    whoami            Fetch authenticated user info for Coder deployment
 
 GLOBAL OPTIONS: 
 Global options are applied to all commands. They can be set using environment

cli/testdata/coder_whoami_--help.golden

@@ -0,0 +1,9 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder whoami
+
+  Fetch authenticated user info for Coder deployment
+
+———
+Run `coder --help` for a list of global options.