Enhance set_attribute to enable attribute copying

**Is your feature request related to a problem? Please describe.**
There's no simple way to copy a value from one attribute to another within the same resource. Current approach is verbose:
```
change fn changeset, _context ->
Ash.Changeset.change_attribute(changeset, :bar, Ash.Changeset.get_attribute(changeset, :foo))
end
```

**Describe the solution you'd like**

```
resource do
actions do
create :create do
accept [:foo]
change set_attribute(:bar, from: :foo)
end
end
end
```

**Describe alternatives you've considered**
Introduce a copy_attribute function for a more declarative syntax:

```copy_attribute(:bar, from: :foo)```

Start with the Ash.Changeset.change_attribute and Ash.Changeset.get_attribute calls in the issue, then trace the set_attribute change entry point used in the resource action DSL. Check the relevant change tests and verify that the proposed from: syntax copies the source attribute value to the destination attribute.

Written by the indexing model from the issue text.