[image_picker] Implement `getMedia` for picking videos and images simultaneously on iOS.

[BeMacized]

This PR implements the getMedia method added to the platform interface in #4675 on iOS, allowing users to pick both images and videos from the gallery simultaneously. It superses #4676 and will remain in draft mode until the changes to the platform interface have been merged and published, so that this PR may be updated.

~~As of right now, this PR still lacks some tests which I will provide later.~~

Relevant issue:

• flutter/flutter#89159

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Hixie]

@BeMacized Is this still on your radar?

[Hixie]

@BeMacized is this still on your radar?

See also: https://github.com/flutter/plugins/pull/5979

[Hixie]

Oops, I see I already asked that once. I'm going to close this PR for now to get it off our review queue, but please don't hesitate to submit a new PR if you have the time to get back to it. In the meantime I'll mention this PR on the issue so that anyone else who wants to pick it up can do so. Thanks again for your contribution!