client/sd: unify the service discovery callbacks within a struct

[JmPotato]

What problem does this PR solve?

Issue Number: ref #8690.

What is changed and how does it work?

Unify the service discovery callbacks within a struct.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

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[ti-chi-bot]

@JmPotato: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-integration-realcluster-test	6908fc1	link	true	/test pull-integration-realcluster-test

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