Declare required fields through skeleton

[exavolt]

Currently, we have support for declaring fields as required through the struct tags, but struct tags are static. We might want to add support for dynamically declaring fields that are required. It should be possible to declare some fields as required through the skeleton similarly to how we provide descriptions for fields.

When we say dynamically, we mean that the requirement is declared by the user, rather than baked into itself.

[exavolt]

Another thing while we are at it. There will be cases where a field is required and its parent is also required, and there will be cases where a field is required but its parent is not. There should be a nice indicator for each.

[exavolt]

Another note. Requirement could be generalized using constraint definition. See #11