Support other types of pluggable transports for user-added bridges

According to @cyberta, the VPN currently supports obfs4 and Snowflake pluggable transports for user-added bridges. We should probably extend this to include WebTunnel at minimum, although from a UX perspective it would be ideal for user-added bridges to have parity with Tor Browser.

That would mean expanding the number of supported pluggable transports to include: