App Clip: Add loading and error states

[SergioEstevao]

| 📘 Part of: #2549 |
|:---:|

Fixes #

This PR introduces a loading and error states to App Clips in order to not show an empty/non-configured clip to users.

For now it just shows a progress bar and then the player or if the episode loading failed an alert saying that an error existed.

Should we consider a small Pocket Casts banner with some text instead?

Loading	Error

To test

• Choose the App Clip target
• Tap run
• Check that on launch you see a loading indicator and then followed up by the player interface
• Stop the app
• Edit the App Clip scheme
• Open the Run Section
• Tap on Argument tabs
• Remove the app clip invocation argument : c2r28q2u
• Start the app clip
• Check if the loading state shows up followed by the error state

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 7.82 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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[danielebogo]

@SergioEstevao
it works as described. I left one comment only about what we discussed in our sync.
Other than that it's all good

[danielebogo]