feat(ocm): gate outbound webapp offers - #5854

This PR is part of the cs3org/OCM-STA#20 and cs3org/OCM-STA#48

Summary

• Offer webapp access on outbound shares only when all four hold: offer_webapp is on (default false), the request carries a webapp candidate, the remote resource type lists blank in webapp-receive, and remote discovery advertises exchange-token. Otherwise every webapp method is dropped and the WebDAV method still goes out.
• Build the access-method list once in freshAccessMethods, before NewShare, and store that same list. The wire request used to be filtered by getProtocols while the share was stored from req.AccessMethods, so storage kept webapp methods the remote never received.
• Use the configured webapp_name and webapp_endpoint on the offer. Empty requirements default to must-exchange-token; supplied ones must be known, non-blank, unique, and match the WebDAV set, otherwise InvalidArgument with no POST and no store. With the offer on, startup requires a non-blank name and an absolute http(s) endpoint with no userinfo, and the offer path re-checks the endpoint with the same rule, so a service built without startup validation drops the webapp rather than offering a scheme-less endpoint. enable_webapp stays the discovery advertisement and doesn't turn the offer on.

flowchart TD
    REQ["req.AccessMethods"] --> F{"freshAccessMethods"}
    F -->|"bad requirements, or two webapps"| STOP["InvalidArgument, no POST, no store"]
    F -->|"offer_webapp off, no candidate, invalid webapp_endpoint, or remote lacks blank or exchange-token"| DROP["webapp dropped, WebDAV kept"]
    F -->|"all four hold, endpoint valid"| KEEP["one webapp, AppName = webapp_name"]
    DROP --> L["one access-method list"]
    KEEP --> L
    L -->|"empty"| STOP
    L -->|"non-empty"| OUT["NewShare, then StoreShare with the same list"]