Removing TrustService from Kernel.Core (to become extension)

[shawncal]

Motivation and Context

The TrustService, along with related types like TrustAwareString showcase a really interesting pattern, but we'd like to repackage them into a SemanticKernal.Extensions.Trust package with a lot more configurability, rather than baking these into the SK Core. With the upcoming support for pre- and post- completion hooks in a Plan, we believe we can build a better solution that leverages those hooks, but is optional, configurable, and fully replaceable if our solution doesn't meet your needs.

Other reasons:

• The TrustAwareString type is at odds with the transition we're making to allow any objects (not just strings) in SK's context variables, function args, and return types.
• Security issues have been raised by architects that indicate that this pattern may give a false sense of security, as the strings in the TrustAwareString objects can be manipulated by static casts, and are therefore unreliable.

Description

• Stripping out all TrustService, TrustAwareString, IsSensitive and related references from the SK.Core and Abstractions packages.

**These will be added back in the form of a new SemanticKernel.Extensions.Trust namespace with optional packages and more features, including an auditable trust chain.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[dmytrostruk]

LGTM!