Object builders allow sneaky mutations of maps inside created POJOs

conjure-java generates Builder classes for plain object types, which contain mutable collections (e.g. HashMap, ArrayList, HashSet). In the .build() method, we pass these mutable collections to the POJO's constructor, which uses Collections.unmodifiableMap(...) to prevent mutations from POJO methods HOWEVER, the builder still has references to the underlying mutable collections, so these can still be mutated!

Foo:
  fields: 
    keys: map<string, integer>

    @Test
    public void sad() {
        Foo.Builder builder = Foo.builder();
        Foo schema = builder.build(); // create a Foo containing an empty 'keys' map

        builder.keys("this call mutates a underlying list", 1234);

        assertThat(schema.getKeys()).isEmpty(); // <- 🔥🔥 this fails
    }

Reported by @j-baker

Start at the generated Builder's .build() method and reproduce the issue with the Foo YAML example and its mutable keys map. Done means mutating the builder after build no longer changes the already-created POJO; add a regression test covering that behavior.