feat: add CAP-85 external executable reference support

[overcat]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 84.34783% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.31%. Comparing base (3aea89f) to head (5836498).

Files with missing lines	Patch %	Lines
src/main/java/org/stellar/sdk/SorobanServer.java	78.00%	4 Missing and 7 partials ⚠️
...ntract/exception/ExternalRefNotFoundException.java	55.55%	1 Missing and 3 partials ⚠️
...ar/sdk/operations/InvokeHostFunctionOperation.java	91.30%	1 Missing and 1 partial ⚠️
...n/java/org/stellar/sdk/scval/ScvExecutableTag.java	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #817      +/-   ##
============================================
+ Coverage     83.23%   83.31%   +0.08%     
- Complexity     1672     1708      +36     
============================================
  Files           232      234       +2     
  Lines          6065     6173     +108     
  Branches        683      702      +19     
============================================
+ Hits           5048     5143      +95     
- Misses          636      641       +5     
- Partials        381      389       +8     

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/Util.java	82.35% <100.00%> (+2.35%)	⬆️
src/main/java/org/stellar/sdk/scval/Scv.java	98.07% <100.00%> (+0.11%)	⬆️
...main/java/org/stellar/sdk/scval/ScvComparator.java	93.92% <100.00%> (+0.39%)	⬆️
...n/java/org/stellar/sdk/scval/ScvExecutableTag.java	90.90% <90.90%> (ø)
...ar/sdk/operations/InvokeHostFunctionOperation.java	92.15% <91.30%> (+3.78%)	⬆️
...ntract/exception/ExternalRefNotFoundException.java	55.55% <55.55%> (ø)
src/main/java/org/stellar/sdk/SorobanServer.java	81.12% <78.00%> (-0.20%)	⬇️

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