Feat: add explicit 'Duration=' parameter to EXT-X-CUE-OUT

[Equipo45]

Description

I have been investigating the problem with DURATION parameters.

Firstly, as you already know it isn't compulsory to establish the parameter as you can see in an example where is use and other which not. The use of the parameter will depend on some key points:

1. Is there another parameter indicating two or more attributes about the ad break EX: #EXT-X-CUE-OUT:DURATION=195.000,BREAKID=103038
2. Does the parser or model used for generating the manifest support it?

I see the change as an improvement for the following reasons:

1. It adds clarity to the parameter that is being introduced by adding it explicitly.
2. Is currently used as a standard by reference companies such as Google.

Regarding all, thank you for making the package and contributing to the open-source community.

A test has been changed for checking the new format

[leandromoreira]

thanks, good work, I believe we must add the parser changes to this pr as well.

[leandromoreira]

I think we should preserve what the what we receive as input.

Meaning, we should expand the parser to include something like cue_out_duration_property to true.

        elif line.startswith(protocol.ext_x_cue_out):
            _parse_cueout(line, state)
            state["cue_out_start"] = True
            state["cue_out"] = True
            if line.contains(somethingsomething):
                   state["cue_out_explicitly_duration"] = True

And then here, we would need to check to see if we'd need to add the Duration or not. WDYT?

[leandromoreira]

Otherwise we'll transform manifests without Duration to have one.

[Equipo45]

Yeah, that's true. I changed it and adapted for parsing it only when DURATION is available.
Also, a new test has been created and some code has been refactored regarding the new state.

[leandromoreira]

thanks for the great work

[mauricioabreu]

Awesome work, thank you!