Add support for env variable JUDGE0_SUPPRESS_PREVIEW_WARNING.

[hermanzdosilovic]

Hi @fkdosilovic,

The intention for this PR is to be able to deliberately disable (suppress) preview warning by either setting env variable JUDGE0_SUPPRESS_PREVIEW_WARNING to any value or by explicitly setting judge0.suppress_preview_warning = True.

Here are examples that work with these changes:

Example 1

# File example.py
import judge0
result = judge0.run(source_code="print('hello, world')", language=judge0.PYTHON)
print(result.stdout)

Any of these would cause the preview warning to be suppressed:

JUDGE0_SUPPRESS_PREVIEW_WARNING=True python3 example.py
JUDGE0_SUPPRESS_PREVIEW_WARNING=1 python3 example.py
JUDGE0_SUPPRESS_PREVIEW_WARNING=asdf python3 example.py
JUDGE0_SUPPRESS_PREVIEW_WARNING= python3 example.py

Example 2

import judge0
judge0.suppress_preview_warning = True
result = judge0.run(source_code="print('hello, world')", language=judge0.PYTHON)
print(result.stdout)

I don't know if this is idiomatic way of doing this in Python. If it's not, please adjust the PR accordingly such that the above can be achieved.

Thank you!

[fkdosilovic]

LGTM