Allow the ability to define the initial timestamp for the packetizer.

[TroyKomodo]

Summary

Currently the timestamp is defined using a random number

https://github.com/pion/rtp/blob/319e780b743768216fcd1facedd8b26ca208ff01/packetizer.go#L40

This doesn't allow for syncing tracks from multiple sources where we can then swap the tracks from the server without repair.

Motivation

I have 4 video sources with different quality settings but they cannot be used with ABR since the initial timestamps do not correspond with each other.

[Sean-Der]

Hey @TroyDota

I am in support of this! I would add a ...Option to the end and have it function like SampleBuilder.Option