Group multisig communication using exn messages with embedded events

[pfeairheller]

This PR includes:

• Addition of Bexter for CESR encoding of variable length data and Pather for SAD Path language encoding.
• Exchanging for creating EXN with support for embedded events.
• New client classes, Exchanges and Groups for performing group operations.

[codecov]

Codecov Report

Merging #100 (7e1d3fb) into development (002bb64) will decrease coverage by 0.36%.
The diff coverage is 75.00%.

@@               Coverage Diff               @@
##           development     #100      +/-   ##
===============================================
- Coverage        80.76%   80.41%   -0.36%     
===============================================
  Files               32       35       +3     
  Lines             3530     3752     +222     
  Branches           797      835      +38     
===============================================
+ Hits              2851     3017     +166     
- Misses             655      710      +55     
- Partials            24       25       +1     

Files Changed	Coverage Δ
src/keri/app/signify.ts	90.71% <48.78%> (-5.52%)	⬇️
src/keri/core/matter.ts	82.90% <70.76%> (-1.37%)	⬇️
src/keri/core/pather.ts	94.11% <94.11%> (ø)
src/keri/core/bexter.ts	97.29% <97.29%> (ø)
src/keri/app/exchanging.ts	97.56% <97.56%> (ø)
src/index.ts	89.58% <100.00%> (+0.22%)	⬆️

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