feat: introduce config object, make service loading optional

[fbiville]

Introduce ImportSpecificationSettings, with a builder API.

It allows users to disable dynamic service loading entirely,
and/or explicitly specify custom validator, source, action
and entity target extension implementation classes.

[fbiville]

Note: I still need to find a way to read ImportSpecificationSettings while deserializing entity target extensions.

[fbiville]

note: I need to add the companion method for this in Plugins, and rely on that method somehow in

import-spec/core/src/main/java/org/neo4j/importer/v1/targets/EntityTarget.java

Lines 28 to 32 in ccfb1ff

	@SuppressWarnings("unchecked")
	private static final List<EntityTargetExtensionProvider<? extends EntityTargetExtension>> EXTENSION_PROVIDERS =
	ServiceLoader.load(EntityTargetExtensionProvider.class).stream()
	.map(provider -> (EntityTargetExtensionProvider<? extends EntityTarget>) provider.get())
	.collect(Collectors.toList());

not sure how to do the latter...

[fbiville]

todo: docs