Add methods to access a single extension to Extensible

[MikeEdgar]

Retrieving an extension via Extensible currently requires the use of getExtensions to acquire the entire extension map, which can then be used to pick the single entry. We should add getExtension(String) and hasExtension(String) methods to allow implementations to provide more efficient ways to access single entries.

Default methods can be provided to make more efficient implementations optional:

default boolean hasExtension(String name) {
    Map<String, Object> map = getExtensions();
    if (map == null) {
        return false;
    }
    return map.containsKey(name);
}

default Object getExtension(String name) {
    Map<String, Object> map = getExtensions();
    if (map == null) {
        return null;
    }
    return map.get(name);
}