Add ability to reuse Rows in encoder

[alamb]

Which issue does this PR close?

Closes #4479

Rationale for this change

When converting many (many!) record batches I would like to avoid the overhead of allocating / zeroing out temporary allocations. See #4479 for more details

What changes are included in this PR?

Add RowConverter::set_target API to reuse an existing Rows when calling RowConverter:: convert_columns

If this is an acceptable API I will write some tests

Are there any user-facing changes?

[tustvold]

Do you have any benchmarks for this

Edit: I also wonder if being able to append to an existing rows might be a more flexible API (I will have a play tomorrow)

[alamb]

Do you have any benchmarks for this

I do not, but I will create some

[alamb]

Edit: I also wonder if being able to append to an existing rows might be a more flexible API (I will have a play tomorrow)

For DataFusion we would also need a way to 'clear' an existing Rows

[alamb]

Will return to this after some profiling results

[tustvold]

#4541 contains a proposal for a slightly more flexible API, that should have similar performance characteristics