Parallel Duration support in config spec

[ChristopherJMiller]

The config spec needs to be updated to define the parallel pull's duration.

Internally, mainframer supported this functionality with PullMode::Parallel(Duration), but with the introduction of #282 this had to be removed due to the inability to serialize/deserialize this enum type in yaml.

To keep compatibility with previous configs in addition to supporting this functionality, I propose an extension to the Pull object itself, adding an optional duration parameter that is only used if pull is defined for the pull mode. If pull is defined without a duration set, we can default to the usual 500ms interval.