feat: Add support for CAP-0015 (FeeBumpTransaction).

[overcat]

#297

[codecov]

Codecov Report

Merging #298 into dev will increase coverage by 0.00%.
The diff coverage is 99.30%.

@@           Coverage Diff            @@
##              dev     #298    +/-   ##
========================================
  Coverage   99.03%   99.03%            
========================================
  Files         107      113     +6     
  Lines        4745     4971   +226     
========================================
+ Hits         4699     4923   +224     
- Misses         46       48     +2     

Flag	Coverage Δ
#unittests	99.03% <99.30%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
stellar_sdk/call_builder/base_call_builder.py	100.00% <ø> (ø)
stellar_sdk/memo.py	100.00% <ø> (ø)
stellar_sdk/server.py	98.52% <83.33%> (-0.97%)	⬇️
stellar_sdk/__init__.py	100.00% <100.00%> (ø)
stellar_sdk/base_transaction_envelope.py	100.00% <100.00%> (ø)
stellar_sdk/fee_bump_transaction.py	100.00% <100.00%> (ø)
stellar_sdk/fee_bump_transaction_envelope.py	100.00% <100.00%> (ø)
stellar_sdk/helpers.py	100.00% <100.00%> (ø)
stellar_sdk/transaction.py	100.00% <100.00%> (ø)
stellar_sdk/transaction_builder.py	100.00% <100.00%> (ø)
... and 12 more