Add config saving to payjoin-cli

[zealsham]

Summary

This Pr adds the ability to save commandline arguement relevant to the config file into a config.toml file in the present directory if anyone exist or into the default xdg-compliant directory .

This pr close #896

[coveralls]

Pull Request Test Coverage Report for Build 17533798681

Details

• 2 of 60 (3.33%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 85.416%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-cli/src/main.rs	2	3	66.67%
payjoin-cli/src/app/config.rs	0	57	0.0%
<!--	Total:	2	60

Totals
Change from base Build 17504088813:	-0.5%
Covered Lines:	8188
Relevant Lines:	9586

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💛 - Coveralls