Avoid stringly typed arguments in payjoin-cli App trait

[nothingmuch]

Minor standalone changes from ongoing refactoring work. A more comprehensive approach might be to allow explicit units to be parsed from the string form of these arguments (see also rust-bitcoin/rust-bitcoin#1786) but personally I think that's undesirable since it complicates payjoin-cli, and as a reference implementation it should prioritize readability and simplicity over convenience/flexibility of the CLI.

[coveralls]

Pull Request Test Coverage Report for Build 12767588652

Details

• 0 of 23 (0.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 60.758%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-cli/src/app/mod.rs	0	1	0.0%
payjoin-cli/src/app/v2.rs	0	2	0.0%
payjoin-cli/src/app/v1.rs	0	4	0.0%
payjoin-cli/src/main.rs	0	16	0.0%

Totals
Change from base Build 12759909420:	0.0%
Covered Lines:	2917
Relevant Lines:	4801

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💛 - Coveralls

[DanGould]

tACK 7c53399. I smoke tested and checked that FeeRate and Amount sent were appropriate, because I don't think these are tested in e2e.rs.

I agree with implicit units with explicit documentation.

This marks progress toward #452 but does not yet complete it because receiver's max_fee_rate is still an Option<u64> but not a proper Option<FeeRate>

[nothingmuch]

This marks progress toward #452 but does not yet complete it because receiver's max_fee_rate is still an Option<u64> but not a proper Option<FeeRate>

Indeed, I did these more than a week ago, and wanted to PR before I forgot, in hindsight I should have reviewed the issue before opening this PR. I will follow up since that's probably also a very simple change.