Add support for source-specific media attributes

[nirbheek]

https://www.rfc-editor.org/rfc/rfc5576, for example, this line:

a=ssrc:3761382600 cname:user1066417796@host-9f2666a9

Is currently parsed as:

Attribute {
    attribute: "ssrc",
    value: Some(
        "3761382600 cname:user1066417796@host-9f2666a9",
    ),
},

But it should be parsed as something like:

SourceAttribute {
    ssrc: 3761382600,
    attributes: [
        Attribute {
            attribute: "cname",
            value: Some("user1066417796@host-9f2666a9"),
        },
    ]
}

And stored under a new field in the Media struct called source_attributes: Vec<SourceAttribute>.