Simplify input for extensible interface reactions

[speth]

Changes proposed in this pull request

• Add a test case for an extensible interface reaction
• Fix the requirement of using an empty rate-constant field and needing the interface- prefix on the implementation of the rate parameterization for extensible reactions on interfaces.

If applicable, fill in the issue number this pull request is fixing

Closes #1620

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.72%. Comparing base (4565a55) to head (6fc7109).

Files	Patch %	Lines
src/kinetics/Reaction.cpp	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1709 +/- ## ========================================== - Coverage 75.72% 75.72% -0.01% ========================================== Files 443 443 Lines 60981 60982 +1 Branches 9557 9558 +1 ========================================== Hits 46178 46178 Misses 11777 11777 - Partials 3026 3027 +1 ```

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