Implemented URI mapping for local file signature

[Qubitza]

As proposed in #15 I introduced the feature to define a URI mapping for local file signatures.

You can either set the actual local file path for a URI or directly pass the content for a defined URI

    ctx.set_uri_mapping({
        let mut map = HashMap::new();
        map.insert(
            "data.json".to_string(),
            UriResource::Data(r#"{"some": "file content"}"#.as_bytes().to_vec()),
        );
        map.insert(
            "another.json".to_string(),
            UriResource::Path(PathBuf::from("some/path/to/another.json")),
        );
        map
    });

The biggest pain point was the fact that the callback functions from xmlsec1 don't provide the context.
This would have allowed to put the mapping into the user_data field.
To solve this challenge, the mapping is written into (and removed afterward) a thread-static variable when sign or validate is invoked.

As I'm not a rust expert feel free to provide improvements.

[Qubitza]

I had to disable MD5 as it was not included in my openssl version.
Feel free to add it back in, if you think MD5 is still a thing.

[Qubitza]

This is the part where the callbacks are set which allow to resolve URI by our own code.