[WIP] Create POC for fuzzing payjoin

[benalleng]

This commit adds fuzzing infrastructure for payjoin

Pull Request Checklist

Please confirm the following before requesting review:

• [x] I have disclosed my use of AI in the body of this PR.
• [x] I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Pull Request Test Coverage Report for Build 18353086055

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.217%

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Totals
Change from base Build 18288677845:	0.0%
Covered Lines:	8954
Relevant Lines:	10632

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💛 - Coveralls