The changes:
1. Add `group_name` to the `channelbag.fcurves.new()` and
`action.fcurve_ensure_for_datablock()` RNA functions.
2. Add `anim_utils.action_ensure_channelbag_for_slot(action, slot)`.
3. Add `channelbag.fcurves.ensure()` RNA function.
This makes it possible to replace this legacy code:
```py
fcurve = action.fcurves.new("location", index=2, action_group="Name")
```
with this code:
```py
channelbag = action_ensure_channelbag_for_slot(action, action_slot)
fcurve = channelbag.fcurves.new("location", index=2, group_name="Name")
```
or replace this legacy code:
```py
fcurve = action.fcurves.find("location", index=2, action_group="Name")
if not fcurve:
fcurve = action.fcurves.new("location", index=2, action_group="Name")
```
with this code:
```py
channelbag = action_ensure_channelbag_for_slot(action, action_slot)
fcurve = channelbag.fcurves.ensure("location", index=2, group_name="Name")
```
Note that the parameter name is different (`action_group` became
`group_name`). This clarifies that this is the name of the group, and
not a reference to the group itself.
This is part of #146586
Pull Request: https://projects.blender.org/blender/blender/pulls/146977
dbcb701eb2