feat: add ContractClient to simplify contract calls.

[overcat]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 1.85185% with 212 lines in your changes missing coverage. Please review.

Project coverage is 77.35%. Comparing base (6104e7c) to head (ed8e494).

Files with missing lines	Patch %	Lines
...org/stellar/sdk/contract/AssembledTransaction.java	0.00%	172 Missing ⚠️
.../java/org/stellar/sdk/contract/ContractClient.java	0.00%	19 Missing ⚠️
...tract/exception/AssembledTransactionException.java	0.00%	3 Missing ⚠️
.../sdk/contract/exception/ExpiredStateException.java	0.00%	2 Missing ⚠️
...ntract/exception/NeedsMoreSignaturesException.java	0.00%	2 Missing ⚠️
...contract/exception/NoSignatureNeededException.java	0.00%	2 Missing ⚠️
...k/contract/exception/NotYetSimulatedException.java	0.00%	2 Missing ⚠️
...ontract/exception/RestorationFailureException.java	0.00%	2 Missing ⚠️
...ract/exception/SendTransactionFailedException.java	0.00%	2 Missing ⚠️
.../contract/exception/SimulationFailedException.java	0.00%	2 Missing ⚠️
... and 2 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #675      +/-   ##
============================================
- Coverage     81.03%   77.35%   -3.68%     
  Complexity     1175     1175              
============================================
  Files           200      212      +12     
  Lines          4444     4658     +214     
  Branches        374      404      +30     
============================================
+ Hits           3601     3603       +2     
- Misses          612      824     +212     
  Partials        231      231              

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/SorobanServer.java	83.73% <ø> (ø)
.../main/java/org/stellar/sdk/TransactionBuilder.java	88.13% <100.00%> (+0.41%)	⬆️
.../sdk/contract/exception/ExpiredStateException.java	0.00% <0.00%> (ø)
...ntract/exception/NeedsMoreSignaturesException.java	0.00% <0.00%> (ø)
...contract/exception/NoSignatureNeededException.java	0.00% <0.00%> (ø)
...k/contract/exception/NotYetSimulatedException.java	0.00% <0.00%> (ø)
...ontract/exception/RestorationFailureException.java	0.00% <0.00%> (ø)
...ract/exception/SendTransactionFailedException.java	0.00% <0.00%> (ø)
.../contract/exception/SimulationFailedException.java	0.00% <0.00%> (ø)
...contract/exception/TransactionFailedException.java	0.00% <0.00%> (ø)
... and 4 more