Make _danger-local-https feature additive

[DanGould]

See: https://doc.rust-lang.org/cargo/reference/features.html#feature-unification

"Rust features should be additive. That is, enabling a feature should not disable functionality, and it should usually be safe to enable any combination of features"

Before this change io::fetch_ohttp_keys changed its signature based on the feature flag. I noticed downstream that this creates a feature flag mess in bindings and it's one opportunity to address tech debt.

This begins to address #392

[coveralls]

Pull Request Test Coverage Report for Build 12167347639

Details

• 7 of 16 (43.75%) changed or added relevant lines in 2 files are covered.
• 26 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.5%) to 61.396%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin/src/io.rs	7	11	63.64%
payjoin-cli/src/app/v2.rs	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
payjoin/src/receive/error.rs	1	1.46%
payjoin/src/send/mod.rs	4	92.48%
payjoin/src/receive/mod.rs	5	90.31%
payjoin/src/psbt.rs	7	46.56%
payjoin/src/io.rs	9	36.17%

Totals
Change from base Build 12163677379:	-0.5%
Covered Lines:	2850
Relevant Lines:	4642

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💛 - Coveralls