Unify app token authentication between Darc and PCS

[oleksandr-didyk]

Contributes to https://dev.azure.com/dnceng/internal/_workitems/edit/6417

Changes introduced:

• introduced new common package for all services - Maestro.Common
• renamed MaestroApiCredential to AppCredential and moved it Maestro.Common
• renamed MaestroApiTokenCredential to ResolvedCredential and moved it to Maestro.Common
• replaced existing *Credential and *TokenCredential implementations in PCS with their Maestro.Common counterparts