Update text provider API to use full AnimationKeypath values

[calda]

This PR replaces the AnimationTextProvider API with a new AnimationKeypathTextProvider API that uses full AnimationKeypath values.

• Fixes AnimationTextProvider should provide the full keypath #1588.
• Fixes CoreAnimation rendering engine changed the behavior of Text providers #1949

AnimationKeypath represents a layer's full path in the animation. For example, a text layer could have an AnimationKeypath like PARENT_A.PARENT_B.text_value.

The old AnimationTextProvider API was implemented inconsistently between the two rendering engines:

• The Main Thread rendering engine always called the text provider with the last component of the text layer keypath (so just text_value).
• The Core Animation rendering engine always called the text provider with the entire keypath (so PARENT_A.PARENT_B.text_value).

Before we even introduced the Core Animation rendering engine, #1588 had previously suggested updating the Main Thread rendering engine to always use the full keypath name. This seems like the best solution, but we just have to make the change in a backwards-compatible way.

This PR introduces a new AnimationKeypathTextProvider API, which takes the entire keypath and has the same behavior across both rendering engines:

/// Protocol for providing dynamic text to for a Lottie animation.
public protocol AnimationKeypathTextProvider: AnyObject {
  /// The text to display for the given `AnimationKeypath`.
  /// If `nil` is returned, continues using the existing default text value.
  func text(for keypath: AnimationKeypath, sourceText: String) -> String?
}

For backwards compatibility we preserve the existing AnimationTextProvider API, with the existing inconsistent behavior, but renamed to LegacyAnimationTextProvider:

/// Legacy protocol for providing dynamic text for a Lottie animation.
/// Instead prefer conforming to `AnimationKeypathTextProvider`.
@available(*, deprecated, message: """
  `AnimationKeypathTextProvider` has been deprecated and renamed to `LegacyAnimationTextProvider`. \
  Instead, conform to `AnimationKeypathTextProvider` instead or conform to `LegacyAnimationTextProvider` explicitly.
  """)
public typealias AnimationTextProvider = LegacyAnimationTextProvider

// MARK: - LegacyAnimationTextProvider

/// Legacy protocol for providing dynamic text for a Lottie animation.
/// Instead prefer conforming to `AnimationKeypathTextProvider`.
public protocol LegacyAnimationTextProvider {
  /// Legacy method to look up the text to display for the given keypath.
  /// Instead, prefer implementing `AnimationKeypathTextProvider.`
  /// The behavior of this method depends on the current rendering engine:
  ///  - The Core Animation rendering engine always calls this method
  ///    with the full keypath (e.g. `MY_LAYER.text_value`).
  ///  - The Main Thread rendering engine always calls this method
  ///    with the final component of the key path (e.g. just `text_value`).
  func textFor(keypathName: String, sourceText: String) -> String
}

In the next major version of Lottie (Lottie 5.0), we could completely delete LegacyAnimationTextProvider.

This PR includes pretty extensive tests capturing the behavior of AnimationKeypathTextProvider, LegacyAnimationTextProvider, and DictionaryTextProvider in both rendering engines.

[calda]

Found and fixed an unrelated bug where the Main Thread rendering engine didn't correctly track and update text layers included in PreCompositionLayers.

[calda]

This was tricky -- I suppose this is why the Main Thread rendering engine didn't previously work like this

[calda]

Adapted from the allKeypaths method above, but just different enough that it didn't seam feasible to consolidate the logic into a single helper

[calda]

Added some more public helpers to this type since it has a bigger role in the public API of AnimationKeypathTextProvider now.

[miguel-jimenez-0529]

LGTM