feat: add SEP-0045 support.

[overcat]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 77.30769% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.98%. Comparing base (2b362c7) to head (7d686ff).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/main/java/org/stellar/sdk/Sep45Challenge.java	78.34%	9 Missing and 46 partials ⚠️
.../sdk/exception/InvalidSep45ChallengeException.java	33.33%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #746      +/-   ##
============================================
- Coverage     81.17%   80.98%   -0.19%     
- Complexity     1322     1382      +60     
============================================
  Files           214      216       +2     
  Lines          5078     5338     +260     
  Branches        436      518      +82     
============================================
+ Hits           4122     4323     +201     
- Misses          700      713      +13     
- Partials        256      302      +46     

Files with missing lines	Coverage Δ
.../sdk/exception/InvalidSep45ChallengeException.java	33.33% <33.33%> (ø)
src/main/java/org/stellar/sdk/Sep45Challenge.java	78.34% <78.34%> (ø)

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