Add checks for unknown values to ValidateConfig() methods

[chrismarget-j]

Attribute values configured like this can never be unknown:

resource "foo" "x" {
  always_lowercase_string = "hello"
}

...But attribute values based on as-yet-unknown values (perhaps those computed later) are another story:

resource "foo" "x" {
  always_lowercase_string = thing_with_upercase_computed_values.x.an_uppercase_string
}

Resources which implement ResourceWithValidateConfig cannot check the value for case because it won't be known when ValidateConfig() is first run.

Attempts to evaluate these attributes will result in the zero value for those types until a future point when the value is calculated. There's little point in trying to validate them in these cases. Worse, the (temporary) zero value might fail validation.

This PR adds escape hatches for the validation routines where unknown values might have had validation attempted.

Closes #124