Added a new .NET project `DigitalData.Auth.Claims` targeting `net8.0`
with metadata for NuGet packaging. The project is configured to
not generate a NuGet package on build.
Introduced the `EnvelopeClaimNames` static class to define strongly-typed
constants for custom JWT claim names specific to envelope receiver tokens.
These include `EnvelopeId`, `EnvelopeUuid`, `ReceiverId`, and
`ReceiverSignature`. Added XML documentation for all constants.
