Accept namespaced Deep Linking settings

[ottenhoff]

Summary

• accept unknown Deep Linking settings only when their property name is an absolute HTTPS URI
• preserve URI-named extension values in the parsed raw claim
• reject ordinary unknown, insecure HTTP, and malformed keys
• cover the four Sakai extension keys reported by CredTrail issue docs: postgres and mysql shipped, D1 requested lti-tool/lti-tool#97

Verification

• npm run lint
• npm run format
• npm run type-check
• npm test -- --run (296 tests)
• npm run build
• npm run smoke-test:dist

Downstream: LongsightGroup/credtrail-private#97