fix: add config for receipts domain

[Theodus]

This adds the config options, chain_id & verifying_contract, required to contruct the EIP-712 domain used when verifying receipts.

[github-actions]

Pull Request Test Coverage Report for Build 6629022963

• 0 of 6 (0.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 42.979%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
service/src/config.rs	0	3	0.0%
service/src/main.rs	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
service/src/config.rs	1	0.0%

Totals
Change from base Build 6590531530:	-0.06%
Covered Lines:	1004
Relevant Lines:	2336

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💛 - Coveralls

[hopeyen]

lgtm! Could you please leave some documentation on available contract addresses one could use, perhaps add a default?

[Theodus]

I think these options should be set explicitly, rather than having a default. I agree we should add a link to an address book for the deployed contracts, as soon as one is available.