Add transformChanges to campaign spec and document it

[mrnugget]

This mirrors the schema changes in sourcegraph/src-cli#398 and is a requirement for merging it.

This also documents transformChanges in the campaign spec YAML reference.

[codecov]

Codecov Report

Merging #16235 (7cb46b7) into main (4b17a4a) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #16235      +/-   ##
==========================================
- Coverage   52.50%   52.49%   -0.01%     
==========================================
  Files        1689     1689              
  Lines       84489    84489              
  Branches     7672     7676       +4     
==========================================
- Hits        44359    44356       -3     
- Misses      36230    36231       +1     
- Partials     3900     3902       +2     

Flag	Coverage Δ
go	51.94% <ø> (-0.01%)	⬇️
integration	29.67% <ø> (-0.02%)	⬇️
storybook	27.96% <ø> (ø)
typescript	53.83% <ø> (+<0.01%)	⬆️
unit	34.67% <ø> (ø)

Impacted Files	Coverage Δ
.../internal/codeintel/resolvers/graphql/locations.go	81.44% <0.00%> (-4.13%)	⬇️
client/web/src/platform/context.ts	68.75% <0.00%> (+3.12%)	⬆️

[mrnugget]

@sourcegraph/campaigns @rrhyne Marking this as ready-for-review since it's a requirement for sourcegraph/src-cli#398. I'll add a how-to and probably extend the docs a bit more in follow-up PRs.

[sourcegraph-bot]

Notifying subscribers in CODENOTIFY files for diff 4b17a4a...7cb46b7.

Notify	File(s)
@LawnGnome	doc/campaigns/references/campaign_spec_yaml_reference.md
@christinaforney	doc/campaigns/references/campaign_spec_yaml_reference.md

[LawnGnome]