Support for Image Sequence Reference in the FCPX adapter

## Feature Request

Like with issue #1092, it would be great if the Final Cut Pro X adapter supported `ImageSequenceReference`'s

## Description

As with FCP 7 XML, some research must be done on support for image sequences in the [FCP-X XML format](https://developer.apple.com/documentation/professional_video_applications/fcpxml_reference)
The `videoSources` attribute of an [asset](https://developer.apple.com/documentation/professional_video_applications/fcpxml_reference/asset) might be a good place to start experimenting(?)

## Context

We should add support for all media reference types in OTIO adapters where possible. So far the `ImageSequenceReference` is implemented in the RV, cmx3600 and xges adapters.

Review issue #1092 and the Apple FCP-X XML reference, starting with the asset's videoSources attribute. Compare the existing ImageSequenceReference implementations in the RV, cmx3600, and xges adapters, then determine the FCP-X representation and verify support with adapter coverage; done means image-sequence references are preserved in FCP-X XML where possible.

Written by the indexing model from the issue text.