Add an option to disable CA pinning

[cisco-dmytro-hissa]

Description

• Duo.cs — Added new public constructor overload DuoApi(ikey, skey, host, user_agent, bool disableCaPinning). Existing 3- and 4-arg constructors flow through with disableCaPinning: false, so default behavior is
  unchanged.
• Duo.cs — Added a caPinningEnabled field (default true) and threaded disableCaPinning through the protected constructor as an optional parameter, keeping the TestDuoApi subclass source-compatible.
• Duo.cs — In GetCertificatePinner(), when pinning is disabled (and SSL validation isn't fully off / no custom roots set), returns the OS-trust-store validator instead of the Duo pinner.
• Duo.cs — UseCustomRootCertificates() now throws InvalidOperationException when CA pinning is disabled, since custom roots are themselves a form of pinning and the two are contradictory. Updated the XML doc to
  note the incompatibility.
• CertificatePinnerFactory.cs — Added GetOsTrustStoreValidator(), which accepts a connection only when the platform's default chain validation passes (sslPolicyErrors == SslPolicyErrors.None) — unlike
  GetCertificateDisabler(), which disables TLS entirely.

How Has This Been Tested?

• OsTrustStoreValidatorTest.TestSuccess — Valid Duo chain is accepted.
• OsTrustStoreValidatorTest.TestUnpinnedRootAccepted — A valid non-Duo root (which the Duo pinner would reject) is accepted — the key behavioral difference from pinning.
• OsTrustStoreValidatorTest.TestFatalSslError — Name-mismatch SSL policy error is rejected (TLS still enforced).
• OsTrustStoreValidatorTest.TestChainErrorRejected — Chain-error SSL policy error is rejected.
• OsTrustStoreValidatorTest.TestNoCertificateRejected — Missing certificate is rejected.
• DisableCaPinningConfigTest.TestCustomRootsWithPinningDisabledThrows — UseCustomRootCertificates() throws InvalidOperationException when pinning is disabled.
• DisableCaPinningConfigTest.TestCustomRootsWithPinningEnabledSucceeds — UseCustomRootCertificates() succeeds and returns the client for chaining when pinning is enabled.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)