feat: improve URI handling by basing it on the libcoap type

[pulsastrix]

This PR rewrites the CoapUri type to work directly with an underlying coap_uri_t object and use the libcoap-provided uri parsing functions.

Additionally the url crate is now an optional dependency, and the CoapUri type now implements FromStr, which allows way easier construction of URIs.

Before (using url crate):

let uri = CoapUri::try_from_url(Url::parse("coap://example.com:4711/foo/bar?answer=42")?)?;

Before (without url crate):

let uri = CoapUri::new(
        CoapUriScheme::Coap,
        Some(CoapUriHost::Name("example.com".to_string())),
        Some(4711),
        Some(vec!["foo".to_string(), "bar".to_string()]),
        Some(vec!["answer=42".to_string()]),,
    );

Now:

let uri: CoapUri = "coap://example.com:4711/foo/bar?answer=42".parse()?;

[github-actions]

Code Coverage Report

Generated for commit 44d605d on Thu Jul 18 15:34:28 UTC 2024.

Package	Line Rate	Health
libcoap/tests/common	88%	✔
libcoap/src/message	37%	❌
libcoap/src/session	53%	➖
libcoap/tests	91%	✔
libcoap-sys	70%	➖
libcoap-sys/src	70%	➖
libcoap/src	44%	❌
Summary	46% (894 / 1946)	❌

[pulsastrix]

Apparently there seems to be some issue with doctests and the unstable --ensure-time cargo flag.

For now, I have disabled these unstable flags and changed the toolchain the tests are run on to stable Rust.

[falko17]

Only very few minor comments, but be aware that I'm not that familiar with this repository.

[falko17]

Apart from the two remaining minor comments, LGTM.