Improve ArrowWriter performance for fixed-length vectors #888

What is the problem the feature request solves?

Writing to Arrow vectors through Comet's ArrowWriter class only uses setSafe on the underlying vectors. This incurs a capacity and resize check for every write. For CometSparkToColumnarExec we know the sizes ahead of time, so this is unnecessary overhead when performing a lot of writes.

Describe the potential solution

For ArrowWriter we should:

1. Expose a way to set the capacities for column vectors. Note that setInitialCapacity on the underlying BaseFixedWidthVector doesn't actually allocate anything, so we may need to call allocateNew as well.
2. Extend ArrowWriter to have unsafe methods that call set on the underlying vectors, probably with assertions that we can elide in release builds.

Additional context

No response