add a Transport config option for the key used to encrypt tokens

[marten-seemann]

Fixes #4065.

[codecov]

Codecov Report

Merging #4066 (98cc6bc) into master (37a3c41) will increase coverage by 0.06%.
The diff coverage is 76.19%.

@@            Coverage Diff             @@
##           master    #4066      +/-   ##
==========================================
+ Coverage   83.51%   83.57%   +0.06%     
==========================================
  Files         148      148              
  Lines       15240    15227      -13     
==========================================
- Hits        12727    12725       -2     
+ Misses       2009     2002       -7     
+ Partials      504      500       -4     

Files Changed	Coverage Δ
interface.go	0.00% <ø> (ø)
transport.go	72.35% <55.56%> (+0.03%)	⬆️
internal/handshake/token_protector.go	48.78% <85.71%> (-1.22%)	⬇️
internal/handshake/token_generator.go	78.95% <100.00%> (+2.76%)	⬆️
server.go	76.29% <100.00%> (+0.36%)	⬆️

... and 1 file with indirect coverage changes

[marten-seemann]

@sukunrt We can use this in libp2p. That way, resumption tokens will survive reboots of the node. (We're currently not saving tokens on the client side, but once we do, the server side is already deployed and it will just work).