guestlinks: implement auth-middleware for Guestlink cookies

[rhafer]

This is currently based on #3609. Needs to be rebased once that on is merged. Until that I'll keep this in draft state.

Needs: opencloud-eu/reva#822

This adds the reva guestlinks authprovider service to the auth-guest service.

Also it adds new a authentication middleware for the Cookies issued by the auth-guest that utilizes the above auth-provider for signing a reva token.

Beware: Currently this gives too broad access (e.g. the authenticated user is able to use graph/v1.0/users?$seach=...)

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 148 complexity

Metric	Results
Complexity	148

View in Codacy

🟢 Coverage 63.38% diff coverage · +0.16% coverage variation

Metric	Results
Coverage variation	✅ +0.16% coverage variation (-1.00%)
Diff coverage	✅ 63.38% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (6ff4794)	90725	22141	24.40%
Head commit (3a55525)	91096 (+371)	22381 (+240)	24.57% (+0.16%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3618)	426	270	63.38%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[aduffeck]

Just for the record: the comment in the description about giving too broad access is no longer true. The guest_link_auth middleware is now limited to these path prefixes:

		"/graph/v1beta1/me/drive/sharedWithMe",
		"/dav/",
		"/remote.php/dav/",
		"/webdav/",
		"/remote.php/webdav/"