maintenance[message-relayer]: make getSentMessages function more testable

[smartcontracts]

Description
Pulls the getSentMessages function out of the MessageRelayer and adds some unit tests. Also adds some additional validation checks to each function to improve our error observability.

[changeset-bot]

🦋 Changeset detected

Latest commit: 28a84f2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@eth-optimism/message-relayer	Patch

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[codecov-commenter]

Codecov Report

Merging #903 (3e58200) into develop (9fea7c9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #903   +/-   ##
========================================
  Coverage    82.21%   82.21%           
========================================
  Files           48       48           
  Lines         1895     1895           
  Branches       303      303           
========================================
  Hits          1558     1558           
  Misses         337      337           

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[tynes]

Can we include the txhash in the error message?

[tynes]

@smartcontracts What is the status of this PR? I notice that its no longer ready for review

[smartcontracts]

@smartcontracts What is the status of this PR? I notice that its no longer ready for review

I'm gonna close this PR since it's largely superseded by #952