Implement plaintext encryption v2

[Menduist]

Not sure yet how to implement this in the builder in a less-easy-to-do-stupid-stuff way

[codecov]

Codecov Report

Merging #821 (a797b5e) into unstable (fe7a69e) will increase coverage by 0.03%.
The diff coverage is 95.45%.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #821      +/-   ##
============================================
+ Coverage     83.61%   83.65%   +0.03%     
============================================
  Files            81       83       +2     
  Lines         14726    14968     +242     
============================================
+ Hits          12313    12521     +208     
- Misses         2413     2447      +34     

Impacted Files	Coverage Δ
libp2p/builders.nim	95.13% <80.00%> (-0.52%)	⬇️
libp2p/protocols/secure/plaintext.nim	98.00% <97.43%> (ø)
libp2p/protocols/connectivity/autonat.nim	84.02% <0.00%> (-2.94%)	⬇️
libp2p/protocols/pubsub/gossipsub.nim	84.51% <0.00%> (-1.36%)	⬇️
libp2p/protocols/pubsub/pubsub.nim	83.16% <0.00%> (-0.87%)	⬇️
libp2p/protocols/pubsub/gossipsub/behavior.nim	87.84% <0.00%> (-0.45%)	⬇️
libp2p/discovery/discoverymngr.nim	97.29% <0.00%> (-0.41%)	⬇️
libp2p/protobuf/minprotobuf.nim	82.17% <0.00%> (-0.15%)	⬇️
libp2p/protocols/connectivity/relay/client.nim	75.00% <0.00%> (-0.12%)	⬇️
... and 6 more