Restructure NegotiateAuthentication implementation (#87930)

Restructure NegotiateAuthentication implementation (#87930)

* Change NegotiateAuthentication implementation to use indirection through NegotiateAuthenticationPal to the actual implementation.

The PAL implementation are merged from different sources to follow the same structure:
- NTAuthentication.Managed.cs -> NegotiateAuthenticationPal.Managed.cs
- NTAuthentication.Common.cs + NegotiateStreamPal.Windows.cs -> NegotiateAuthenticationPal.Windows.cs
- NTAuthentication.Common.cs + NegotiateStreamPal.Unix.cs -> NegotiateAuthenticationPal.Unix.cs

This split allows to delete ContextFlagsPal, SafeDeleteNegoContext, and SafeFreeNegoCredentials abstractions that were used in NegotiateStreamPal.

* Unify impersonation level validation between Windows and Unix pllatforms

* Split managed NTLM and managed SPNEGO implementations; add UseManagedNtlm switch on Unix platforms

* Remove debug cruft

* Fix couple of errors in managed SPNEGO

* Remove debug print

* Fix message sequence in managed NTLM; remove unused method

* Fix fallbacks on macOS GSSAPI

* Cleanup and fallbacks for missing NTLM, GSSAPI

* Adjust tests to assume that NTLM is always available on Unix

* Don't claim NTLM support on Browser

* Revert "Don't claim NTLM support on Browser"

This reverts commit 87d0c56f67a5269d9b334ab17887338ac4cfe49b.

* Attempt to fix the browser tests

* Revert "Attempt to fix the browser tests"

This reverts commit 91d7ce289a7274b682803b1d5dfdf418a5c2120c.

* Browser test suppression

* Respect UseManagedNtlm=false on platforms without NTLM GSSAPI provider

* Update src/libraries/System.Net.Security/src/System/Net/NegotiateAuthenticationPal.Unix.cs

* Revert all the fallback code paths, System.Net.Security.UseManagedNtlm has to be enabled explicitly; NativeAOT on Linux Bionic does that by default because it doesn't have GSSAPI and native shim