Fix - Allow ExtendProgramChecked in CPI

[Lichtso]

Problem

See solana-foundation/solana-improvement-documents#164 (comment).

Feature Gate Issue: https://github.com/anza-xyz/feature-gate-tracker/issues/87

Summary of Changes

Allows invoking loader-v3 ExtendProgramChecked in CPI.
Rekeys the feature enable_extend_program_checked.

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 82.7%. Comparing base (b56095d) to head (67a9cf3).
Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6245   +/-   ##
=======================================
  Coverage    82.7%    82.7%           
=======================================
  Files         843      843           
  Lines      377600   377606    +6     
=======================================
+ Hits       312608   312622   +14     
+ Misses      64992    64984    -8     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.