feat(cli): add formatting options to coder whoami (#19970)

This addresses a long-standing gripe of mine: to get your logged in
username you would have to do
```bash
coder whoami | awk '{print $9}'
```

This allows you to do:

```
coder whoami -o json | jq -r '.username'
```

or

```
coder whoami -f table -c username
```

cli/testdata/coder_whoami_--help.golden

@@ -1,9 +1,16 @@
 coder v0.0.0-devel
 
 USAGE:
-  coder whoami
+  coder whoami [flags]
 
   Fetch authenticated user info for Coder deployment
 
+OPTIONS:
+  -c, --column [URL|Username] (default: url,username)
+          Columns to display in table output.
+
+  -o, --output text|json|table (default: text)
+          Output format.
+
 ———
 Run `coder --help` for a list of global options.