Add assign custom button actions for button sets and service templates

[ohhAndy]

Depends on ManageIQ/manageiq#23953

Adds assign_custom_button resource actions for custom button sets and service templates, wires the new actions into config/api.yml

@miq-bot add-label enhancement
@miq-bot add-reviewer @Fryguy
@miq-bot assign @Fryguy

[miq-bot]

Checked commit ohhAndy@05023f0 with ruby 3.3.10, rubocop 1.88.2, haml-lint 0.76.0, and yamllint 1.37.1
4 files checked, 16 offenses detected

spec/requests/custom_button_sets_spec.rb

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spec/requests/service_templates_spec.rb

• ❗ - Line 841, Col 132 - Layout/SpaceInsideHashLiteralBraces - Space inside } detected.
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