parent block ID API

[bw-solana]

Problem

This API is needed for Alpenglow when votor crate gets integrated (see #7802)

Summary of Changes

• Add API to get the block ID of the parent from the child bank.
• Add test for this API.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.0%. Comparing base (7eb0faa) to head (e7d0666).

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #7812     +/-   ##
=========================================
- Coverage    83.0%    83.0%   -0.1%     
=========================================
  Files         812      812             
  Lines      357194   357209     +15     
=========================================
- Hits       296736   296730      -6     
- Misses      60458    60479     +21     

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