CCM-8579: Switch from util types to api client types

[ClareJonesBJSS]

Description

Rewrites the API spec to take advantage of anyOf/oneOf to avoid having all channel-specific properties as optional properties on a single object, and removes util types in favour of the generated ones.

• Switches generators as the current one is unsupported and had some bugs I could only work around by structuring things weirdly and/or a post generation transform step
• Switches to unions over enums

Context

On creating the letter create/update pages, I went through a lot of silly mistakes and stray properties that TS could have caught, along with little discrepancies where the generated types and nhs-notify-web-template-management-utils types/validations had diverged. Consolidating on TemplateDto gives a lot more confidence that types actually reflect reality at runtime and will be accepted by the API

Testing

Manually tested SMS/email/NHS app template creation, editing, copying, deleting and submitting and all executed successfully with no visible regression. Letters seem to be working well so far for my follow on work and Alex's backend work at #340

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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