Add --external-smt2-solver for custom solver path or custom options

[tautschnig]

Similar to --external-sat-solver: enable calling SMT2 solvers with a non-default name or possibly wrapping a solver to pass additional options.

Fixes: #8720

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

❌ Patch coverage is 51.72414% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.93%. Comparing base (b6caec4) to head (b8a2281).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_dec.cpp	33.33%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8726      +/-   ##
===========================================
- Coverage    79.93%   79.93%   -0.01%     
===========================================
  Files         1698     1698              
  Lines       187681   187698      +17     
  Branches        73       73              
===========================================
+ Hits        150022   150034      +12     
- Misses       37659    37664       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 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.