feat: executive summary gains `scenario` and `scenario_source` specificity

[jdhoffa]

It turns out that, while many of the prep_ functions do utilize a parameterized scenario argument, we had just been relying on the default 1.5C-Unif in most cases, for most functions.

This PR does a few things, all broadly related to parameterizing the scenario and scenario_source selection:

• render_executive_summary gains argument scenario_source_selected
• all functions in all 2024 templates gain specification for scenario_selected and scenario_source_selected (as applicable)
• defaults to the 2022 values (GECO2021 and 1.5C-Unif) for backward compatibility. (I don't know how necessary that really is, but hey ho, why not follow good practices)

[codecov[bot]]

Codecov Report

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

Project coverage is 9.69%. Comparing base (cb14a97) to head (6fdbb8c).

Files	Patch %	Lines
R/render_executive_summary.R	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #314 +/- ## ======================================== - Coverage 9.69% 9.69% -0.01% ======================================== Files 27 27 Lines 2186 2187 +1 ======================================== Hits 212 212 - Misses 1974 1975 +1 ```

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