feat(aggregator): Add support for multiple signers in input

[aasseman]

Lets the aggregator accept receipts or input RAVs that are signed with other keys.
Helpful with dealing with the transition when rotating keys, or making sure that we can accept receipts signed by other signing keys belonging to the same Sender (multiple locations with different keys for example).

Fixes #205

[coveralls]

Pull Request Test Coverage Report for Build 7764301533

• -11 of 53 (79.25%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 73.81%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tap_aggregator/src/server.rs	19	20	95.0%
tap_aggregator/src/aggregator.rs	23	26	88.46%
tap_aggregator/src/main.rs	0	7	0.0%

Totals
Change from base Build 7762886114:	0.1%
Covered Lines:	1364
Relevant Lines:	1848

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💛 - Coveralls

[github-actions]

🤖 Cargo Audit Report 🤖

(Empty means OK! 👍)

[gusinacio]

LGTM