Coerce types on read

[aykut-bozkurt]

COPY FROM parquet is too strict when matching Postgres tupledesc schema to the parquet file schema.
e.g. INT32 type in the parquet schema cannot be read into a Postgres column with int64 type.
We can avoid this situation by casting arrow array to the array that is expected by the tupledesc
schema, if the cast is possible. We can make use of arrow-cast crate, which is in the same project
with arrow. Its public api lets us check if a cast possible between 2 arrow types and perform the cast.

To make sure the cast is possible, we need to do 2 checks:

1. arrow-cast allows the cast from "arrow type at the parquet file" to "arrow type at the schema that is
   generated for tupledesc",
2. the cast is meaningful at Postgres. We check if there is an explicit cast from "Postgres type that corresponds
   to the arrow type at Parquet file" to "Postgres type at the tupledesc".

With that we can cast between many castable types as shown below:

• INT16 => INT32
• UINT32 => INT64
• FLOAT32 => FLOAT64
• LargeUtf8 => UTF8
• LargeBinary => Binary
• Struct, Array, and Map with castable fields, e.g. [UINT16] => [INT64] or struct {'x': UINT16} => struct {'x': INT64}

NOTE: Struct fields must match by name and position to be cast.

Closes #67.

Part of #49.

[codecov]

Codecov Report

Attention: Patch coverage is 93.44614% with 275 lines in your changes missing coverage. Please review.

Project coverage is 92.00%. Comparing base (518a5ac) to head (c69fd25).

Files with missing lines	Patch %	Lines
src/pgrx_tests/copy_from_coerce.rs	87.64%	116 Missing ⚠️
src/pgrx_tests/copy_type_roundtrip.rs	90.78%	89 Missing ⚠️
src/arrow_parquet/schema_parser.rs	77.12%	43 Missing ⚠️
src/pgrx_tests/common.rs	90.22%	22 Missing ⚠️
src/arrow_parquet/arrow_to_pg.rs	96.32%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
- Coverage   92.83%   92.00%   -0.83%     
==========================================
  Files          62       70       +8     
  Lines        7645     8769    +1124     
==========================================
+ Hits         7097     8068     +971     
- Misses        548      701     +153     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.