Add private claims to receipt sample code

[achamayou]

The logging sample ought to be extended to support private claims and illustrate what's suggested under https://microsoft.github.io/CCF/main/use_apps/verify_tx.html#application-claims.

The documentation should be updated to clarify that the private claims are commitments rather than encrypted data.

The sample also ought to illustrate the use of #4194 once it is available.