feature(core): Add introspection powers to the pipeline control API

[TheElectronWill]

Example (from the tests):

let request = request::list_elements(ElementListFilter::kind(ElementKind::Source));
let list = rt
    .block_on(handle.send_wait(request, TIMEOUT))
    .expect("list request failed");
assert_eq!(
    list,
    vec![ElementName::from_str(ElementKind::Source, "test", "simple_source")]
)

This is needed to write proper integration tests for the fix of #132.

Progresses towards #79.