Add ENCRYPTED PRIVATE KEY and OpenVPN patterns to PrivateKeyDetector

[Vignesh-Kathirkamar]

• Please check if the PR fulfills these requirements

• Tests for the changes have been added

• Docs have been added / updated

• All CI checks are green

• What kind of change does this PR introduce?

This adds two missing private key patterns to PrivateKeyDetector

• What is the current behavior?

PrivateKeyDetector does not flag BEGIN ENCRYPTED PRIVATE KEY (PKCS#8 encrypted format) or BEGIN OpenVPN Static key V1 files. These are detected by pre-commit/pre-commit-hooks' detect-private-key but not by detect-secrets. (Related: #70)

• What is the new behavior (if this is a feature change)?

Both patterns are now included in the PrivateKeyDetector denylist and will be flagged as secrets.

• Does this PR introduce a breaking change?

No. Existing behavior is preserved; this only adds detection for two additional patterns.

• Other information:

These patterns were identified by comparing detect-secrets' denylist against pre-commit-hooks' detect-private-key (which this plugin was originally derived from, per the source file header). As per #70's discussion, it was agreed that encrypted keys should still be flagged since passphrase strength cannot be assumed.