Update trait/specification(s) to indicate required properties for `preflight`

[foundrytom]

What

Document any required trait properties for preflight

Why

OpenAssetIO recently changed the signature to Preflight, as a result some specifications may mandate that trait properties are required for preflight.

[elliotcmorris]

It might be best to assume everything is required and only indicate optional properties.

[elliotcmorris]

We have to go through all the properties in traits.yml and decide, "Is this property required."

Theres an argument that if we're adding a "required" field into the yaml, we should also traitgen that to be nice, but we don't have to, there's already a precedent with usage.

[feltech]

I think this issue has been misunderstood. The current GUIDELINES are pretty clear that trait properties are required by default.

I think this was specifically to call out which trait properties are required for preflight - not which traits are required for resolve(!)

I've updated the description to hopefully avoid misinterpretation again.