Add F-Curve management functions on ChannelBags
(`channelbag.fcurves.xxx`) that are very similar to the legacy Action
functions `Action.fcurves.xxx`.
```python
channelbag = strip.channelbags.new(slot)
fcurve = channelbag.fcurves.new("rotation_quaternion", index=1)
assert channelbag.fcurves[0] == fcurve
channelbag.fcurves.remove(fcurve)
channelbag.fcurves.clear()
```
Pull Request: https://projects.blender.org/blender/blender/pulls/124987