Integrate NPY copy into the existing COPY pipeline

[ray6080]

We support npy as a file format to be copied into node tables, specifically into fixed size array columns.
Currently, npy copy goes through NodeCopyExecutor, which is separate from our query processor. While copying of csv and parquet files goes through the query processor (in the issue title, I call it "COPY pipeline").
For simplicity of code, we should move npy copy to go through the query processor too.
Specifically, we need to remove NodeCopyExecutor, and move the logic of copying npy files to CopyNode and ReadFile operators.

Two main issues need to be addressed for this change:

1. Change NpyReader to read a DataChunk at a time, so it can fit into ReadFile opeartor. The use of mmap can be removed since we read sequentially, which should be fine from the perspective of IO performance.
2. NPY copy follows the syntax of COPY BY COLUMN, thus one file corresponds to one column, which is quite different from CSV and Parquet files. CSV and Parquet files cover all columns in a single file. So CopyNode should be aware of that npy is copying a DataChunk into one column at a time, instead of all columns.