remove const in LMS_serialize_key and add `is_locked` to OQS_SIG_STFL… #907

Job	Run time
Check upstream code is properly integrated	1m 23s
Check code formatting	1m 11s
Check that code passes a basic build before starting heavier tests	0s
linux_arm_emulated	0s
linux_intel	0s
	2m 34s

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