[TRACKING ISSUE] custom load balancer declarative configuration

The tracking issue for the Java side PR.

**custom load balancer declarative configuration**

See https://github.com/hazelcast/hazelcast/pull/17415 for details.

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Fixes https://github.com/hazelcast/hazelcast/issues/17228

Changes:

- "custom" balancer type was introduced (as suggested)
- XML, YAML config loading supports new balancer type
- Spring bean class name or reference is also supported for "custom" type

I'm aware that there have to be seperate PRs for other branches. Are you guys able to cherry pick this change onto 4.0.z and 4.z branches or do I have to open separate PRs? (this will take some time)

Can you help review this first?

Start with pull request #17415 and the referenced issue #17228; this tracking issue contains no repository files or tests to inspect. Confirm whether the requested custom balancer configuration belongs in this Python client and whether the Java-side changes need separate branch updates; done means the stated XML, YAML, and Spring configuration support is covered.

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