add ClientTransportPlugin configuration to tor-service-defaults-torrc by default

Please add ClientTransportPlugin configuration to /usr/share/tor/tor-service-defaults-torrc by default. What I mean by that...

tor-browser/Browser/TorBrowser/Data/Tor/torrc-defaults contains:

## fteproxy configuration
ClientTransportPlugin fte exec ./TorBrowser/Tor/PluggableTransports/fteproxy.bin --managed

## obfs4proxy configuration
ClientTransportPlugin obfs2,obfs3,obfs4,scramblesuit exec ./TorBrowser/Tor/PluggableTransports/obfs4proxy

## meek configuration
ClientTransportPlugin meek exec ./TorBrowser/Tor/PluggableTransports/meek-client-torbrowser -- ./TorBrowser/Tor/PluggableTransports/meek-client

## snowflake configuration
ClientTransportPlugin snowflake exec ./TorBrowser/Tor/PluggableTransports/snowflake-client -url https://snowflake-reg.appspot.com/ -front www.google.com -ice stun:stun.l.google.com:19302

For /usr/share/tor/tor-service-defaults-torrc I suggest to add:

## fteproxy configuration
ClientTransportPlugin fte exec /usr/bin/fteproxy --managed

## obfs4proxy configuration
ClientTransportPlugin obfs2,obfs3,obfs4,scramblesuit exec /usr/bin/obfs4proxy

(Left out meek and snowflake, because these are not yet in packages.debian.org. legacy/trac#13160 (moved) legacy/trac#19409 (moved))

Why?

• Improves usability. One step of configuration less. Fewer mistakes can be made. The user has no longer to add the ClientTransportPlugin line.
• Using the canonical recommendation.
• To be on par with Tor Browser.
• ClientTransportPlugin does not have any effect as long as not adding a Bridge line.
• ClientTransportPlugin lines do not change often.
• ClientTransportPlugin can still be overwritten in /etc/tor/torrc by the user.