feat(cli): add shell completions (#14341)

cli/testdata/coder_speedtest_--help.golden

@@ -6,9 +6,8 @@ USAGE:
   Run upload and download tests from your machine to a workspace
 
 OPTIONS:
-  -c, --column string-array (default: Interval,Throughput)
-          Columns to display in table output. Available columns: Interval,
-          Throughput.
+  -c, --column [Interval|Throughput] (default: Interval,Throughput)
+          Columns to display in table output.
 
   -d, --direct bool
           Specifies whether to wait for a direct connection before testing
@@ -18,8 +17,8 @@ OPTIONS:
           Specifies whether to run in reverse mode where the client receives and
           the server sends.
 
-  -o, --output string (default: table)
-          Output format. Available formats: table, json.
+  -o, --output table|json (default: table)
+          Output format.
 
       --pcap-file string
           Specifies a file to write a network capture to.