Fix checksum flag and tests

[ashvp]

Metadata
Reference Issue: Fixes #875
New Tests Added: Yes
Documentation Updated: No
Change Log Entry: “Add configurable checksum (digest) checking to downloads”

Details:
This PR implements the feature requested in Issue #875, allowing users to configure whether MD5 digest validation should be performed when downloading data.

What this PR implements

1. Adds a keyword-only parameter check_digest to _send_request, defaulting to True to maintain current behavior.
2. Integrates the flag with the existing checksum validation logic.
3. Updates global configuration (openml.config.check_digest) so users can disable digest checking across all downloads.
4. Adds a dedicated test suite validating:
   • Correct checksum acceptance
   • Mismatch detection
   • Digest-skipping behavior when the flag is turned off

Why this change is needed
The client currently always validates checksums, which can cause performance overhead or failures in workflows where:

• files are repeatedly downloaded in tight loops,
• local caching is trusted,
• or when users intentionally bypass digest validation for speed.
  Making digest checking optional improves flexibility without compromising default safety.

How to reproduce the issue being solved
Before:
Calling _send_request(..., md5_checksum="wrong") always raises OpenMLHashException.

After:
Calling _send_request(..., md5_checksum="wrong", check_digest=False) succeeds without raising an exception.