implement account with seed in system program

[drbh]

This PR implements create_account_with_seed similar to

https://github.com/solana-labs/solana-web3.js/blob/44f32d9857e765dd26647ffd33b0ea0927f73b7a/src/system-program.ts#L743-L771

[drbh]

@michaelhly please let me know if I should make any changes before merge

[kevinheavey]

I'm not a maintainer but you'll want to write a test for this in tests/unit/test_system_program.py. Should be simple to imitate an existing test

[drbh]

@kevinheavey thanks for the advice! I've added a unit test test_create_account_with_seed and implemented the decode_create_account_with_seed function that was missing from the original commit

[codecov]

Codecov Report

Merging #101 (d1a3e08) into master (7bf0ee0) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   88.02%   88.14%   +0.12%     
==========================================
  Files          32       32              
  Lines        2130     2135       +5     
==========================================
+ Hits         1875     1882       +7     
+ Misses        255      253       -2     

[michaelhly]

Looks good. But you need to run make fmt. Thanks!