refactore(source): move SuitableType to endpiont package

[ivankatliarchuk]

What does it do ?

Move SuitableType away from source.

related: #6248

Motivation

1. Reusability without import cycles — endpoint is a foundational package imported by source, plan, provider, controller, etc. By putting SuitableType there, any package that already imports endpoint can use it without importing source. Before the move, any non-source package needing this logic would have to duplicate it or create a cycle.
2. Logical placement — SuitableType operates purely on DNS record type constants (RecordTypeA, RecordTypeAAAA, RecordTypeCNAME) that live in the endpoint package. Having the function that maps a target string to those constants sit next to the constants themselves is the natural home for it. Keeping it in source was an accident of where it was first needed, not where it belongs conceptually.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

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[coveralls]

Pull Request Test Coverage Report for Build 22621694429

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 53 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 79.15%

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Files with Coverage Reduction	New Missed Lines	%
utils.go	1	98.44%
pihole/clientV6.go	21	92.45%
gateway.go	31	85.78%

Totals
Change from base Build 22540459008:	-0.02%
Covered Lines:	16035
Relevant Lines:	20259

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💛 - Coveralls