[Python] Add a function telling whether Parquet encryption is available

[asfimport]

It would be nice to have a function, e.g. pyarrow.parquet.is_encryption_available(), to be able to query whether Parquet encryption is available in the PyArrow build.

(currently, one would have to import the relevant module and catch the ImportError)

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [Python][Parquet] High-level Python API for Parquet encryption (is related to)

_{Note: This issue was originally created as ARROW-15825. Please see the migration documentation for further details.}