Cache tokens for service principal

[isra-fel]

Description

This PR adds support for token cache when logging in with a service principal.
It reduces the (often unnecessary) network traffic to re-get the token for doing other operations, and retrieves token from cache instead.

Checklist

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD select appropriate branch. Cmdlets from Autorest.PowerShell should go to generation branch.
• SHOULD make the title of PR clear and informative, and in the present imperative tense.
• SHOULD update ChangeLog.md file(s) appropriately
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense. Add changelog in description section if PR goes into generation branch.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD have approved design review for the changes in this repository (Microsoft internal only) with following situations
  • Create new module from scratch
  • Create new resource types which are not easy to conform to Azure PowerShell Design Guidelines
  • Create new resource type which name doesn't use module name as prefix
  • Have design question before implementation
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT introduce breaking changes in Az minor release except preview version.
• SHOULD NOT adjust version of module manually in pull request

[isra-fel]

Hi @BethanyZhou please review this PR but do not merge. I will make a signed build and run smoke test to double check. Thank you.

[BethanyZhou]

Looks good to me