Add OAuth2 in Backend #298

Should be done with OPA

https://www.openpolicyagent.org/docs/latest/

Policies should be defined externally of ScriptBee WebApp

[ Request with Bearer Token ]
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1. JWT Validation (Signature, exp, aud)
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2. IClaimsTransformation
├─ Read External ID (sub / oid)
├─ Memory Cache Hit? ──(Yes)──► Attach InternalUserId Claim
└─ Cache Miss? ────────(No)───► Find/Create in Users DB ──► Cache ──► Attach Claim
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3. Custom Authorization Handler
└─ Check: Does InternalUserId have Permission X in Route's ProjectId?
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4. Minimal API Endpoint Executes