The SDK has a mechanism that effectively makes it possible to declare an
attribute as being _conditionally_ required, which is not a concept that
Terraform Core is aware of.

Since this mechanism is in practice only used for a small UX improvement
in prompting for these values interactively when the environment variable
is not set, we avoid here introducing all of this complexity into the
plugin protocol by just having the provider selectively modify its schema
if it detects that such an attribute might be set dynamically.

This then prevents Terraform Core from validating the presence of the
argument or prompting for a new value for it, allowing the null value to
pass through into the provider so that the default value can be generated
again dynamically.

This is a kinda-kludgey solution which we're accepting here because the
alternative would be a much-more-complex two-pass decode operation within
Core itself, and that doesn't seem worth it.

This fixes #19139.

Since the SDK's schema system conflates attributes and nested blocks, it's
possible to state some nonsensical schema situations such as:

- A nested block is both optional but has MinItems > 0
- A nested block is entirely computed but has MinItems or MaxItems set

Both of these weird situations are handled here in the same way that the
existing helper/schema validation code would've handled them: by
effectively disabling the MinItems/MaxItems checks where they would've
been ignored before.

the MinItems/MaxItems

This is no longer effective and should not be used in any new schema.