fix: optional field validation

[Gozala]

Fixes #123

Overview

• Adds optional parameter to the parse function used by validator to parse capabilities.
• .select method which selects matching proofs from delegation chain passes option:true to parser so nb fields are never required in delegations.
• derives of the capability infers type nb fields as optionals.
• derives in cap.derive({ to, derives }) infers nb fields as optionals.
• .delegate(opts).capabilities[0].nb infers fields as optionals.
• .invoke(opts).capabilities[0].nb infers fields as defined in schema.

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I hate introducing more utility types to make this work, but unfortunately there was no simple way to fix all the problems. It is possible to rework validator such that we would be able to remove bunch of these utility types, however it is non trivial change and I'd rather do it later on when there's less urgency.

[alanshaw]

This looks ok to me, although with the caveat that I don't have a a very deep level of understanding of ucanto yet...