feat: add functions to send assets to contract address without relying on Stellar RPC.

[overcat]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 97.74436% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.06%. Comparing base (4d7e298) to head (123840c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../main/java/org/stellar/sdk/TransactionBuilder.java	98.48%	1 Missing and 1 partial ⚠️
...ain/java/org/stellar/sdk/operations/Operation.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #677      +/-   ##
============================================
+ Coverage     77.35%   80.06%   +2.71%     
- Complexity     1175     1210      +35     
============================================
  Files           212      212              
  Lines          4658     4790     +132     
  Branches        404      406       +2     
============================================
+ Hits           3603     3835     +232     
+ Misses          824      703     -121     
- Partials        231      252      +21     

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/StrKey.java	90.41% <ø> (ø)
...ain/java/org/stellar/sdk/operations/Operation.java	89.00% <0.00%> (ø)
.../main/java/org/stellar/sdk/TransactionBuilder.java	95.28% <98.48%> (+7.15%)	⬆️

... and 7 files with indirect coverage changes

[overcat]

stellar/quickstart#667