Enable dynamic changesetTemplates and steps outputs

[mrnugget]

This updates the campaign spec schema to the one in sourcegraph/src-cli#424 and should be merged before.

I'm going to add documentation in a separate PR so we don't tie these together.

[codecov]

Codecov Report

Merging #17243 (b0f6020) into main (a46292d) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main   #17243   +/-   ##
=======================================
  Coverage   51.98%   51.98%           
=======================================
  Files        1707     1707           
  Lines       84999    84999           
  Branches     7563     7510   -53     
=======================================
+ Hits        44186    44189    +3     
+ Misses      36903    36900    -3     
  Partials     3910     3910           

Flag	Coverage Δ
go	51.03% <ø> (+<0.01%)	⬆️
integration	30.53% <ø> (ø)
storybook	30.05% <ø> (ø)
typescript	54.29% <ø> (ø)
unit	34.80% <ø> (ø)

Impacted Files	Coverage Δ
...nal/campaigns/resolvers/changeset_apply_preview.go	59.82% <0.00%> (+0.85%)	⬆️
.../internal/codeintel/resolvers/graphql/locations.go	81.44% <0.00%> (+2.06%)	⬆️