Refactor to use eventuals for allocations and signers (v2)

[Jannis]

This is the final PR to replace #54.

It turns allocation monitoring and attestation signer creation into eventuals, which simplifies the logic and makes it easy to chain different values that change over time together.

[github-actions]

Pull Request Test Coverage Report for Build 6552034814

• 298 of 347 (85.88%) changed or added relevant lines in 8 files are covered.
• 4 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-1.5%) to 40.785%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
common/src/attestations/signers.rs	50	51	98.04%
service/src/server/mod.rs	0	2	0.0%
common/src/attestations/signer.rs	84	88	95.45%
service/src/query_processor.rs	4	10	40.0%
service/src/tap_manager.rs	0	7	0.0%
service/src/main.rs	0	14	0.0%
common/src/allocations/monitor.rs	157	172	91.28%

Files with Coverage Reduction	New Missed Lines	%
common/src/allocations/monitor.rs	1	91.88%
common/src/attestations/signers.rs	1	96.67%
service/src/main.rs	1	0.0%
service/src/tap_manager.rs	1	0.0%

Totals
Change from base Build 6519277925:	-1.5%
Covered Lines:	925
Relevant Lines:	2268

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💛 - Coveralls

[aasseman]