Add JS-API for validation of delegations

[przydatek]

Add validate_delegation_and_get_principal-helper that thoroughly validates the given delegation and upon success returns a Principal that identifies the user (cf. description of the helper for details).

There are some outstanding tasks for follow-up PRs (marked with TODO in comments):

• add a "deeper" expiration check for of the delegation (to check the "freshness" of the delegation)
• replace the added of code for computing representation independent hash with a crate (the crate exists, but is currently incomplete)
• add tests in JS for the wrapped functionality

[frederikrothenberger]

LGTM, thanks! Left one comment.

[przydatek]

Thanks!