Added the `DigitalData.Auth.Claims` project to the solution,
including its build and debug configurations. Updated
`DigitalData.Auth.API.csproj` to reference the new project
and incremented version numbers to 1.4.0.
Enhanced `Program.cs` with a new JWT signature handler for
`EnvelopeReceiverSecretDto`, generating claims for envelope
and receiver-specific data. Added `DirectorySearchService`
to the service collection, configured via `DirectorySearchOptions`.
