feat: add --header-command flag (#9059)

This allows specifying a command to run that can output headers for
cases where users require dynamic headers (like to authenticate to their
VPN).

The primary use case is to add this flag in SSH configs created by the
VS Code plugin, although maybe config-ssh should do the same.

cli/testdata/coder_--help.golden

@@ -62,6 +62,11 @@ variables or flags.
           Additional HTTP headers added to all requests. Provide as key=value.
           Can be specified multiple times.
 
+      --header-command string, $CODER_HEADER_COMMAND
+          An external command that outputs additional HTTP headers added to all
+          requests. The command must output each header as `key=value` on its
+          own line.
+
       --no-feature-warning bool, $CODER_NO_FEATURE_WARNING
           Suppress warnings about unlicensed features.