Expose Clock from builder and implement in Spring Boot AutoConfiguration

[deximat]

Brief, plain english overview of your changes here

• Added clock(Clock) method to SchedulerBuilder so users can provide their own clock for Scheduler
• In order not to have to think about Waiter caching old Clock I moved Waiter creation to build method, so it always uses latest Clock from builder field
• Had to build Waiter manually in TestHelper for ManualScheduler.

Note: This changes current behavior, but I think it's a bug.
Currently Waiter in ManualScheduler is using SystemClock instead of newly set clock, I changed that because Waiter is now created in the end so it will always use up-to date Clock instance, in case this is desired behavior for some reason, let me know and I will fix it.

Reminders

• Added/ran automated tests
• Update README and/or examples
• Ran mvn spotless:apply

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cc @kagkarlsson

[deximat]

Now user can just define Clock bean and it will automatically be used by AutoConfiguration.

[kagkarlsson]

Thank you for contributing!

What was the use-case here? Override clock used in tests that uses Spring to instantiate the Scheduler?