Enable setting batch size in `add_columns`

[tonyf]

Default batch size for lance may cause OOM depending on the size of a row and available system memory

[westonpace]

Yes, we should make this configurable. However, in the meantime, you can use LANCE_DEFAULT_BATCH_SIZE for spots where we don't make it configurable.

[tonyf]

Is num_threads configurable via an environment variable as well?

[tonyf]

nvm, looks like it is via LANCE_IO_THREADS

[westonpace]

There is also a LANCE_CPU_THREADS. The difference between LANCE_IO_THREADS and LANCE_CPU_THREADS is described in more detail here.