[border-agent] organize `CoapDtlsSession` method definitions (#11142) #199

Job	Run time
channel-manager-csl	3m 12s
expects	31m 31s
packet-verification-1-1-on-1-4	11m 14s
packet-verification-low-power	9m 22s
thread-1-4-gcc-m32	11m 18s
thread-1-4-gcc-m64	11m 20s
thread-1-4-posix	9m 41s
thread-1-4-clang-m32	9m 58s
thread-1-4-clang-m64	10m 13s
upload-coverage	21s
	1h 48m 10s

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