More unit tests for frontend domain handlers

[Shaddoll]

What changed?

• Improve coverage for frontend domain handlers
• Move domain handlers to a separate file
• Move request validation to requestValidator
• Fix request validation for Register and Update Domain requests

Why?
Improve code quality

How did you test it?
unit tests

Potential risks

Release notes

Documentation Changes

[davidporter-id-au]

nice!

[codecov]

Codecov Report

Attention: Patch coverage is 87.42138% with 20 lines in your changes missing coverage. Please review.

Project coverage is 78.97%. Comparing base (e170bd0) to head (20d697d).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
service/frontend/api/domain_handlers.go	77.27%	14 Missing and 6 partials ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
service/frontend/api/handler.go	72.93% <ø> (+1.73%)	⬆️
service/frontend/api/request_validator.go	100.00% <100.00%> (ø)
service/frontend/api/domain_handlers.go	77.27% <77.27%> (ø)

... and 8 files with indirect coverage changes

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