[Dotnet] Fix for imports/usings reserved names replacement

[andrueastman]

This PR closes #1799

It resolves compile issues after the introduction of the Microsoft.Graph.IdentityGovernance namespace to the beta metadata.
As the namespace introduces a task navigation property, it results to conflict between the generated namespace name and generated Types.

Changes include: -

• Fixes an issue where if namespaces are renamed, the corresponding using statements referencing the namespaces were not renamed
• Adds the ReplaceReservedNamespaceTypeNames function to allow for namespaces to replaced with a different format than models as a namespace Microsoft.Graph.IdentitityGovernance.TaskObject conflict with trying to reference a type called TaskObject as the compiler would not know whether to reference the type or the Namespace.

Generation run - https://dev.azure.com/microsoftgraph/Graph%20Developer%20Experiences/_build/results?buildId=84434&view=results
V1 changes - microsoftgraph/msgraph-sdk-dotnet@e317dda
Beta changes - microsoftgraph/msgraph-beta-sdk-dotnet@0cdc5cb

[baywet]

Thanks for taking this on!

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

80.0% Coverage
0.0% Duplication