Content scanner (Element Pro): scan timeline events for unsafe content (#7159)

* Simplify how event content with captions are displayed

The caption component will now be added in `TimelineItemEventContentView` instead of separately in each `TimelineItem__View` component

* Make sure `CoilMediaFetcher` methods to fetch media throw errors

We need to catch them later to check if the content scanner marked some thumbnails or medias as invalid/unsafe in `AsyncImage` instances

* Add a `ContentScannerService` that allows us to manually scan a list of medias in an event and then update and store the results

The results take the form of `ContentValidationValue`, stored in a `ContentValidationState` instance tied to the event

* Create `EventContentValidationCache` to keep the `ContentValidationStates` for events in a room

* Expose `EventContentValidationCache` to Compose using `LocalEventContentValidationState`

Also add a couple of helper functions to avoid having to manually remember the states

* Use the new components to display invalid content in the timeline items

* Add `blurhash` to most media items so we can use it while validating the thumbnails/medias

* Fix custom layout issues with screenshot and UI tests

* Also check the validation state for replied-to events in the composer

* Fix and add tests

* Handle unrecoverable content validation errors too: display a 'not found' UI

* Improve the performance of `EqualWidthColumn` for the single item case

* Update screenshots

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Co-authored-by: ElementBot <android@element.io>

features/contentscanner/impl/build.gradle.kts

@@ -32,4 +32,5 @@ dependencies {
     testImplementation(libs.test.truth)
     testImplementation(libs.test.turbine)
     testImplementation(projects.libraries.matrix.test)
+    testImplementation(projects.tests.testutils)
 }