Add dnstt pluggable transport

As Tor Browser will have dnstt PT support (tpo/anti-censorship/pluggable-transports/trac#40001), we need to:

• Add dnstt to the pluggable transports table - https://tb-manual.torproject.org/circumvention/
• Add dnstt when we list all PTs supported in Tor Browser - https://tb-manual.torproject.org/circumvention/

This should be done only when dnstt is shipped in Tor Browser. Ref: tpo/applications/tor-browser#40859