Add getTypes

[charjr]

We may need to consider how to make this consistent:

null would imply type has not been specified, thus can be any type.

In 3.1 null is a type, so not specifying type means that null is acceptable.

But in 3.0 null is not a type, nullable is a keyword. So if type is unspecified, but nullable is set to false... null is not acceptable.

openapi-reader/src/ValueObject/Valid/V30/Schema.php

Lines 150 to 154 in 88427b2

	if ($this->nullable) {
	$result[] = Type::Null;
	}
	return empty($result) ? null : $result;

Therefore, not specifying type has different meaning, depending on the version.

Proposed Solution

We have getTypes() return all acceptable types.

Advantages

• We get a version agnostic "this is what it can be"
  • We get a version agnostic "this can|can't be null"

Disadvantages

We cannot rely on getTypes() to tell if it was unspecified.
To know if a type was specified, we would have to look at the property $type.
Thus we would want something like this when using it:

if (isset($schema->type)) {
    $types = $schema->getTypes();
    // do something...
}