Consistent ordering of subtypes in schema

[big-andy-coates]

Add a test such as the following to SchemaGeneratorTest.java:

@Test
 void shouldConsistentlyOrderSubTypes() {
        // Given:
        @JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
        class BaseType {}
        class SubType1 extends BaseType {}
        class SubType2 extends BaseType {}

        generator.registerSubTypes(List.of(BaseType.class));

        // When:
        final String result = generator.generateSchema(BaseType.class);

        // Then:
        assertThat(
                result,
                containsString(
                        ""
                                + "oneOf:"
                                + lineSeparator()
                                + "- $ref: '#/definitions/SubType1'"
                                + lineSeparator()
                                + "- $ref: '#/definitions/SubType2'"));
    }

and this test may or may not fail, depending on the ordering of the sub types. The issue is that the order is defined by Jackson's StdSubtypeResolver.collectAndResolveSubtypesByClass(), which is storing the found NamedType's in a HashMap ... which leads to inconsistencies...

The work around would be to install a 'fixed' subclass resolver that orders the returned set of named types.

The longer term fix is to have mbknor-jackson-jsonschema sort the returned collection by type name.