Parse event logs with ethers.js

[bh2smith]

As suggested in #182 (comment) (from PR #182) we introduce transaction log parsing via ethers library with appropriate Contract artifacts. Namely, we introduce dependencies on @cowprotocol/contracts which contain the abi files for GPv2Settlement and iERC20 and suffice for us to decode relevant event data from settlement transactions. For us these are Transfer, Settlement and Trade.

We construct Event types out of each which only keep the parts that matter for our purposes (namely TradeEvent having owner as the only relevant field).

Unit Tests are more-or-less the same (except we had to change test addresses to checksum addresses) and we also had to add some non-trivial data to our mock TradeEventLog since ethers expects to be able to parse even the irrelevant, to us, fields of a Trade.

Note that this also allowed us to remove a utility method that was previously used to manually decode raw transaction data.

Test Plan

Existing Tests

[fedgiac]

I like the changes! There are two comments I think are important before merging, otherwise it looks good.

[gentrexha]

Approving, but take it with a grain of salt as I have no previous ts knowledge/experience.

[socket-security]

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Pull request alert summary

Issue	Status
Install scripts	✅ 0 issues
Native code	✅ 0 issues
Bin script shell injection	✅ 0 issues
Unresolved require	✅ 0 issues
Invalid package.json	✅ 0 issues
HTTP dependency	✅ 0 issues
Git dependency	✅ 0 issues
Potential typo squat	✅ 0 issues
Known Malware	✅ 0 issues
Telemetry	✅ 0 issues
Protestware/Troll package	✅ 0 issues

📊 Modified Dependency Overview:

➕ Added Package	Capability Access	+/- Transitive Count	Publisher
@cowprotocol/contracts@1.3.2	network	+7	cowprotocol_dev
ethers@6.0.3	network, filesystem	+6	ricmoo