refactor: decouple StrKey muxed helpers from xdr wrappers

[overcat]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.42%. Comparing base (2d4d06c) to head (edb13fb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/java/org/stellar/sdk/StrKey.java	44.44%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #780      +/-   ##
============================================
- Coverage     81.50%   81.42%   -0.09%     
  Complexity     1473     1473              
============================================
  Files           220      220              
  Lines          5527     5530       +3     
  Branches        565      568       +3     
============================================
- Hits           4505     4503       -2     
- Misses          715      717       +2     
- Partials        307      310       +3     

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/Address.java	83.00% <100.00%> (ø)
src/main/java/org/stellar/sdk/MuxedAccount.java	82.85% <100.00%> (-0.48%)	⬇️
src/main/java/org/stellar/sdk/StrKey.java	94.11% <44.44%> (-2.20%)	⬇️

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