feat: add --ssh-host-prefix flag for "coder ssh" (#16088)

This adds a flag matching `--ssh-host-prefix` from `coder config-ssh` to
`coder ssh`. By trimming a custom prefix from the argument, we can set
up wildcard-based `Host` entries in SSH config for the IDE plugins (and
eventually `coder config-ssh`).

We also replace `--` in the argument with `/`, so ownership can be
specified in wildcard-based SSH hosts like `<owner>--<workspace>`.

Replaces #16087.

Part of https://github.com/coder/coder/issues/14986.

Related to https://github.com/coder/coder/pull/16078 and
https://github.com/coder/coder/pull/16080.

docs/reference/cli/ssh.md

@@ -20,6 +20,15 @@ coder ssh [flags] <workspace>
 
 Specifies whether to emit SSH output over stdin/stdout.
 
+### --ssh-host-prefix
+
+|             |                                         |
+|-------------|-----------------------------------------|
+| Type        | <code>string</code>                     |
+| Environment | <code>$CODER_SSH_SSH_HOST_PREFIX</code> |
+
+Strip this prefix from the provided hostname to determine the workspace name. This is useful when used as part of an OpenSSH proxy command.
+
 ### -A, --forward-agent
 
 |             |                                       |