feat: Add AtServerEvent interface and AtSignPKChangedEvent class. Enables better reset handling #2026

Job	Run time
functional_tests_at_onboarding_cli (at_onboarding_cli_functional_tests)	2m 23s
build_and_test (at_lookup)	43s
build (at_contact)	35s
build_and_test (at_chops)	1m 11s
build_and_test (at_onboarding_cli)	45s
build (at_server_status)	34s
build_and_test (at_commons)	44s
build_and_test (at_utils)	39s
build_and_test (at_cli_commons)	48s
build_and_test (at_policy)	48s
	9m 10s

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