[rust-0.44] No way to mutate MutableStructArray

[utkarshgupta137]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Issue description

In #19123, the mut_values() method was removed from MutableStructArray. As far as I can tell, there is no way to add elements to MutableStructArray anymore. I've gone through all the APIs available in the docs & also gone through the code base. The only workaround seems to be to copy the MutableStructArray code & re-add mut_values.
All other similar MutableXYZArray still provide one way or another (either mut_values() or push()) to mutate the data. Also, there are two places in the code base where a similar struct is used & both of them still provide mut_values() method.

Expected behavior

My personal preference would be to re-add the mut_values() method. If that is something the devs want to do away with, we at least need push() & preferably extend().

Installed versions

polars = { version = "0.44", default-features = false, features = ["bigidx", "dtype-datetime", "dtype-date", "dtype-time", "dtype-duration", "dtype-u8", "dtype-u16", "dtype-i8", "dtype-i16", "performant", "ipc", "polars-io"] }
polars-core = { version = "0.44", default-features = false }

[ritchie46]

I think we are working towards removing it. We don't use it, so it will be removed.