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GH-33987: [R] Support new dplyr .by/by argument (#35667)

### Rationale for this change

Implement the `.by` argument for `mutate`, `summarise`, `filter` and `slice_*` family.

### What changes are included in this PR?

The `.by` argument that matches `dplyr` has been added to some functions.

Most of the internal functions, such as `compute_by`, are copied from the existing `dplyr` backends, `dbplyr` and `dtplyr`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #33987

Authored-by: SHIMA Tatsuya <ts1s1andn@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>