Allow non-root OIDC issuer

[kschibli]

My OIDC endpoint is not at the root and I couldn't use discover_jwks via from_oidc because it would strip the path of the issuer.

Before:

issuer: 'https://example.com/myissuer/'
result: 'https://example.com/.well-known/openid-configuration'

After:

issuer: 'https://example.com/myissuer/'
result: 'https://example.com/myissuer/.well-known/openid-configuration'

I checked, and having the discovery url not at the root seems to be supported by the standard:

https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfigurationRequest

[cduvray]

Yeah you are absolutely right this is a bug, but I think, to make this work properly we should replace "join".

This assert is passing:

assert_eq!(
        Url::parse("https://example.net/a/b")?
            .join(".well-known/openid-configuration")?
            .to_string(),
        "https://example.net/a/.well-known/openid-configuration"
    );

as you see 'b' disappears from the url.

I think this should be something like:

url
        .path_segments_mut()?
        .pop_if_empty()
        .extend(&[".well-known", "openid-configuration"]);

[cduvray]

Ok, I will merge this and I will remove join after that.