Improve parameters types and docs

[lmuntaner]

Motivation

Improve the documentation and validation of the parameters expected by requestVerifiablePresentation.

Changes

• Wrap new URL with a try/catch to provide better feedback tot he user.
• Change types string for Principal for the credential subject and the issuer canister id.
• Add types for better documentation and developer experience.
• Add ts-doc comments and file comment.

Tests

• Test that onError is called when identityProvider is not a valid URL.
• Change params type in tests.
• Add canisterId to the expected request sent to IdentityProvider through window post message.

[frederikrothenberger]

LGTM, thanks!