Add support for dnstt

This is the main issue for all the things that need to happen in order to add support for dnstt PT. Please feel free to edit/move this issue if needed.

According to the discussion we had in this week's meeting, the following should happen:

• Setup a test dnstt bridge (Implemented: http://eweiibe6tdjsdprb4px6rqrzzcsi22m4koia44kc5pcjr7nec2rlxyad.onion/tpo/anti-censorship/pluggable-transports/trac/-/issues/40001#note_2788292)
• add support for the PT spec in the dnstt client and server
• Add support for dnstt in BridgeDB and rdsys
• iOS and Android clients - WIP
• Get instruction on how to setup a dnstt bridge for different OSes - tpo/web/community#263
• Add support for dnstt in Tor Browser
• Write Tor Browser user documentation - tpo/web/manual#118
• Add default dnstt bridges as failover to Meek and Snowflake