Add Sequential requestor api domains

[qroll]

Changes

• Add api domains for Sequential requestor endpoints, using class-validator and class-transformer
• Include additional setup for runtime decorators
• Note: webhook typings will be in a separate PR
• [delete] branch

Changelog entry

• Add Sequential requestor api domains

[qroll]

also requesting to update the repository settings to require approval before merging 👀

[weili-govtech]

also requesting to update the repository settings to require approval before merging 👀

cc: @keithtxw