Format mint URL (lowercase hostname)

[callebtc]

• Convert hostname to lowercase, keep path in original case
• Remove new (not used? Use from instead)

[thesimplekid]

Should this return an error instead of an empty string?

[callebtc]

The test in mod_rotate_keyset passes an empty string, so this part just reproduces the old code. What do you think?

[thesimplekid]

I think it should return an error. We can change the test MintConfg to have a valid url with:

        let config = MintConfig::<'_> {
            mint_url: "http://example.com",
            ..Default::default()
        };

cdk/crates/cdk/src/mint/mod.rs

Line 1572 in 1cd80de

let config: MintConfig = Default::default();

like we do in the test above it:

cdk/crates/cdk/src/mint/mod.rs

Lines 1532 to 1536 in 1cd80de

	async fn mint_mod_new_mint() -> Result<(), Error> {
	let config = MintConfig::<'_> {
	mint_url: "http://example.com",
	..Default::default()
	};

[thesimplekid]

I think we should just remove this From and move this logic into the FromStr below having this from doesn't add much as if a user has a type Into<String> they can just call MintUrl::from_str(&their_type.to_string())? this avoids hiding the error case here by returning an empty string.