Create Feature Compliance Manifest for ably-flutter

[QuintinWillison]

For motivation, see Future Direction for the SDK Manifests.

Acceptance criteria:

Scope of work under this issue:

• the manifest should have a conformed name and location, as decided in the work done under Define and document standard filename and location for feature manifest files in SDK repositories features#6
• A CI check must be added that uses our new GitHub Action, validating and render SDK manifests against the canonical features list (see internal Jira story SDK-2403)
• A CI step must be added that pushes the rendered manifest view to sdk.ably.com using our SDK Upload Action

┆Issue is synchronized with this Jira Task by Unito

[QuintinWillison]

To explain my addition of a suffix to this issue's title, despite its superfluous nature given the GitHub context, it helps us internally when viewing this issue under the Jira epic that it's sync'd to internally. Sorry.