Add get_processed_sibling_instruction syscall

[jackcmay]

Problem

There are scenarios where instructions (top-level or via CPI) would like to know what the last instruction processed was. Some examples are verification of assert instructions or to check for memo instructions. Programs can already check tx level instructions via the Sysvar1nstructions1111111111111111111111111 but there is no way to get information about what inner instructions have been processed.

Summary of Changes

Add a syscall that allows a program to query what sibling instructions have been successfully processed.

/// Returns a sibling instruction from the processed sibling instruction list.
///
/// The processed sibling instruction list is a reverse-ordered list of
/// successfully processed sibling instructions. For example, given the call flow:
///
/// A
/// B -> C -> D
/// B -> E
/// B -> F
///
/// Then B's processed sibling instruction list is: [A]
/// Then F's processed sibling instruction list is: [E, C]

Summary of Changes

Backport of #22859

Fixes #

[CriesofCarrots]

Just gave this a whirl on https://github.com/CriesofCarrots/solana-program-library/commits/finish-memo-extension
Worked great

[codecov]

Codecov Report

Merging #22956 (c1729ee) into v1.9 (c3890ad) will decrease coverage by 0.0%.
The diff coverage is 40.7%.

@@            Coverage Diff            @@
##             v1.9   #22956     +/-   ##
=========================================
- Coverage    81.2%    81.1%   -0.1%     
=========================================
  Files         531      531             
  Lines      147632   147930    +298     
=========================================
+ Hits       119987   120108    +121     
- Misses      27645    27822    +177