add command to derive child key from master public key #78

Job	Run time
go_sec_covenant_signer	1m 14s
lint_test / build	45s
docker_pipeline / prepare-metadata	1s
lint_test / gosec	2m 30s
lint_test / integration-tests	6m 58s
lint_test / lint	29s
lint_test / unit-tests	1m 20s
lint_test / check-mock-gen	0s
docker_pipeline / docker_build (linux/amd64, ubuntu-24.04)	3m 3s
docker_pipeline / docker_build (linux/arm64, ubuntu-24.04-arm64)	3m 2s
docker_pipeline / merge_ecr	0s
docker_pipeline / merge_dockerhub	0s
	19m 22s

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