Partially migrate policy scenario workflow

[yiyi1991]

This is a draft PR to make the review of changes in policy scenario workflows easier.
It is a prerequisite for the BMT development (issue #483 ) and has been split out accordingly.

• budgets/emission targets (EN steps with multiple options)
• Calling ScenarioRunner on private branch (NPi*, NDC*, LTS(glasgow)) and options to add *_forever
• config to be stored on public branch

How to review

• Read the diff and note that the CI checks all pass.
• Make sure changes in the ENGAGE workflow and function in tools do not affect previous projects that call them
• The documentation makes sense.

PR checklist

• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

❌ Patch coverage is 14.10256% with 134 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.9%. Comparing base (1f7fe08) to head (3812cd4).

Files with missing lines	Patch %	Lines
message_ix_models/tools/policy.py	16.3%	77 Missing ⚠️
message_ix_models/project/engage/workflow.py	8.4%	54 Missing ⚠️
message_ix_models/tools/remove_emission_bounds.py	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #491     +/-   ##
=======================================
- Coverage   73.3%   72.9%   -0.4%     
=======================================
  Files        301     301             
  Lines      24429   24573    +144     
=======================================
+ Hits       17914   17933     +19     
- Misses      6515    6640    +125     

Files with missing lines	Coverage Δ
message_ix_models/tools/add_emission_trajectory.py	80.0% <100.0%> (ø)
message_ix_models/workflow.py	96.1% <100.0%> (+<0.1%)	⬆️
message_ix_models/tools/remove_emission_bounds.py	50.0% <0.0%> (-5.6%)	⬇️
message_ix_models/project/engage/workflow.py	30.5% <8.4%> (-10.2%)	⬇️
message_ix_models/tools/policy.py	21.1% <16.3%> (-37.2%)	⬇️

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