Mitigate breaking change of optional parameter change for ModelFactory

[live1206]

Fixes #3596.

This PR introduces SignatureType, it holds three portions of code:

• current
• custom
• baseline contract
  current union custom compare with previous contract outputs the changeset, with it we can apply different rules with

It provides

• OverloadingMethods: for now represents the missing methods in baseline contract due to additional optional parameters in the new contract, this is causing breaking changes
• MethodsToSkip: represents the methods already defined in custom code, the writer should skip them while writing to the output

This feature is only enabled for Mgmt.

• For DPG, there is no GA version yet, ApiCompatVersion is not set in .csproj
• For HLC, there are quite some hand-written/custom code, which is not supported by TypeFactory.TryCreateType since CsharpType does not represent them properly for now.

[live1206]

Pending on Azure/azure-sdk-for-net#41322