Only discover complex types when configured through pre-convention model configuration

[roji]

In the current bits, when a CLR type is configured as a complex type in one place, it's picked up as a complex type by convention in other places as well; so a CLR type can be used as a complex type in many places in the model, but only explicitly configured as a complex type once. This means that if the one explicit configuration is removed for some reason, all others become broken in various ways.

Instead, we can add an API to pre-convention model configuration to configure a CLR type as complex, and otherwise require that users explicitly configure.

Note the interaction with #31275, which is about automatically configuring nested complex types as complex. Implementing the behavior above removes the problematic part raised in #31275 (comment).