OpenAPI to expose attribute max length and whether a property is required

[andrus]

Let's add extra pieces of information to Agrest OpenAPI integration (and presumably, the underlying Ag model):

• Whether the property is required (OpenAPI schema.maxLength for Strings)
• Max length of the attribute (OpenAPI schema.required)

[alevshunov]

We have noticed that openapi lost the attribute format="time" for type="string" where db time of a field is a time.

[andrus]

@alevshunov I just opened a separate task. Seems like an easy fix.