Pin transitive dependencies to mitigate CVEs

[cgillum]

Dependency management:

• Resolve CVEs by pinning transitive dependencies via explicit references to Azure.Core, Azure.Identity, Microsoft.IdentityModel.JsonWebTokens, System.IdentityModel.Tokens.Jwt, and System.Text.Json in DurableTask.SqlServer.csproj and TestDBGenerator.csproj.
• Added Directory.Packages.props to the solution file to use central package management.

Code cleanup:

• Removed the AsyncQueue class from AsyncQueue.cs as it is no longer needed (this allowed us to also remove dependency on System.Threading.Channels).
• Fixed nullable reference types warnings found by updated dependency analyzers

Version updates:

• Updated the patch version from 1.5.0 to 1.5.1 in common.props