[FEAT] Support for per-operation grouping within fsummarise/fmutate akin to dplyr's .by argument

[wgrundlingh]

What aspect of the package is your request related to?
dplyr introduced per-operation grouping support via a .by argument within functions like summarise and mutate in 2023 (dplyr 1.1.0), thereby avoiding explicit group_by() and ungroup() calls in a pipeline. A similar option for fsummarise and fmutate would be great!

Describe the solution you'd like
Instead of

<some pipeline> %>%
  fgroup_by(<group vars>) %>%
  fsummarise(<summary vars>) %>%
  fungroup() %>%
  <more pipeline>

allow for

<some pipeline> %>%
  fsummarise(
    .by = <group vars>,
    <summary vars>
  ) %>%
  <more pipeline>

and similarly for fmutate.

Additional context
This would allow for a virtual drop-in replacement of summarise or mutate functions into existing code, with much smaller pipeline modifications.