Move Share Requirements out from protocols

[KrausMatthias]

If the requirements must be identical for each protocol via which a share is provided, we should move them out from NewShare.protocol[*].requirements to NewShare.requirements instead of duplicating them.

In my opinion the protocols should only contain information relevant to the individual transport and non-supported protocols should be safe to ignore.

OCM-API/IETF-OCM.md

Lines 1137 to 1145 in a5b5da6

	- REQUIRED requirements (array of strings) -
	The requirements that the sharee MUST fulfill to
	access the Resource. The requirements MUST at least include
	`must-exchange-token`. If multiple protocols are present in the
	share payload, the requirements for the different protocols MUST
	agree. For example, if a webapp share is sent in the same payload
	as a webdav share, both protocols MUST carry the same
	requirements, and both requirement arrays MUST include
	`must-exchange-token`.

Are there any implementations which use the Share Requirements so far? The requirements where only defined for WebDAV in previous versions.