feat(FEC-10686): move startTime config from playback to sources

[dan-ziv]

Description of the Changes

move startTime to the sources config so it will take place only on the a specific source.

CheckLists

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• test are passing in local environment
• Travis tests are passing (or test results are not worse than on master branch :))
• Docs have been updated

[OrenMe]

@dan-ziv is it possible to set this later and not in hls new instance creation? sounds more reasonable to do this

[dan-ziv]

@OrenMe If you remember we did this optimization along time ago since we wanted to use the hls config to achieve chunk downloading from the startTime point. The other option is to seek to the start point and we manage this in the load() as well (in case we didn't get startTime earlier).

[OrenMe]

I actually wrote this and then so the old PR and approved - but forgot to delete the comment :-)