Use IntoUrl for more ergonomic function signatures

[DanGould]

Re: #513
Inspired by reqwest::IntoUrl

After some consideration, I'm not sure if the goal really should be to remove url types from the public API. That crate is ancient and has a stable release cadence and conservative MSRV targets (1.63 even with the latest) that we can track.

However, I still think the IntoUrl interface makes our typestate machines easier to work downstream with for the tradeoff of added complexity in our crate.

In order to graduate from DRAFT I'd need new error types for extract functions that include an into_url::Error encapsulating variant. Before I do that work, I'm seeking a concept ack from @spacebear21 or otherwise a rejection of the idea, presumably on the grounds of too much complexity.

Note, in payjoin-cli We're still validating URLs in configuration, and using the url::Url abstraction in function signatures makes more sense than becoming more loose for testing.

The greatest advantage of this change may accrue to downstream FFI users, who I imagine use their native language's Url type and conversion to a stringly type that payjoin-ffi handles using IntoUrl rather than forcing them to use the payjoin::Url re-export and error handling.

[coveralls]

Pull Request Test Coverage Report for Build 13360441514

Details

• 62 of 91 (68.13%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.09%) to 79.041%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin/src/send/v2/error.rs	0	3	0.0%
payjoin/src/io.rs	4	9	44.44%
payjoin/src/into_url.rs	27	36	75.0%
payjoin/src/receive/v2/error.rs	1	13	7.69%

Totals
Change from base Build 13355590759:	-0.09%
Covered Lines:	4039
Relevant Lines:	5110

---

💛 - Coveralls

[spacebear21]

cACK, I don't think the complexity is too wild, and I agree 100% with this point:

This reduces a step for [downstream implementations] and unifies our error handling for added
internal complexity, which seems like the express purpose of the library.

Linking to reqwest::IntoUrl in the module-level documentation or even the commit message would help clearly point to the source of inspiration for our approach.

[DanGould]

Rather than link to the reqwest IntoUrl docs at the module level, I made the type pub (since it's in function signatures) and documented the inspiration in the type's docstring. If it were documented at the private module level there'd be no way to find it.

[DanGould]

Blocking this until #522 is addressed

[spacebear21]

ACK b3f0966