Expose API to configure filename of token cache

[xiangyan99]

Intro:
Customers expect the identity can be shared between Azure PowerShell and Azure CLI. It allows the user to log into Azure once using either Azure PowerShell or Azure CLI. Customers can then switch script language according to their needs. It's very important to end users when there's a feature gap between Azure PowerShell and Azure CLI.

Proposal:
Currently, the token cache file is specified by Azure.Identity. Azure.Identity should expose an API to allow the client to configure both the file name and location. With this API, Azure PowerShell and Azure CLI can determine the common location and share the token cache with each other.

[azure-sdk]

Label prediction was below confidence level 0.6 for Model:ServiceLabels: 'App Configuration:0.56844413,Extensions:0.13713013,API Management:0.05199336'