Move the sources of GenericThreadBorderRouterDelegate to src/platform #91326

Job	Run time
Build framework (macos-release, -sdk macosx -configuration Release)	17m 24s
Build framework (ios-release, -sdk iphoneos -configuration Release)	10m 37s
Build framework (tvos-debug, -sdk appletvos -configuration Debug)	8m 57s
Build framework (watchos-debug, -sdk watchos -configuration Debug)	14m 21s
Run framework tests (asan, -enableAddressSanitizer YES -enableUndefinedBehaviorSanitizer YES)	46m 27s
Run framework tests (tsan, -enableThreadSanitizer YES)	36m 51s
Run framework tests (leaks, ENABLE_LEAK_DETECTION=1)	31m 19s
	2h 45m 56s

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