Auto-generate test PEM files at runtime for OpenSearch sink mTLS tests #6826

Description

The mTLS client certificate tests added in #6755 commit static PEM files to the test resources directory (test-client-cert.pem, test-client-key.pem, test-client-ca.pem, test-client-ca-key.pem). Per review feedback from @sb2k16 and @dlvenable , these should be generated programmatically at test runtime using BouncyCastle, following the pattern already established in ConnectionConfiguration_ServerTest.createTrustStoreFromPem().

What needs to change

• Remove the committed PEM files from data-prepper-plugins/opensearch/src/test/resources/
• Generate client CA, client certificate, and client key at test startup using BouncyCastle JcaX509CertificateConverter and JcaPKCS8Generator
• Update ConnectionConfiguration_ServerTest.ClientCertificateConfiguration and ConnectionConfigurationTests to use the generated certs
• Update the integration test OpenSearchClientCertIT to generate certs in its @BeforeAll setup

Context

This was identified during the review of #6755. The PR was merged with the static PEM files to unblock the feature, with agreement to follow up.

References

• PR: Add client certificate authentication (mTLS) to OpenSearch sink #6755
• Review comment: Add client certificate authentication (mTLS) to OpenSearch sink #6755 (comment)