Refactor retry logic and add special behavior for IMDS

[Avery-Dunn]

This PR refactors HTTP retry logic by splitting it into classes that implement a new interface, similar to what was done in MSAL Node and .NET:
AzureAD/microsoft-authentication-library-for-js#7614
AzureAD/microsoft-authentication-library-for-dotnet#5231

The following changes were made:

• A new IRetryPolicy interface, defining methods to determine if a certain HTTP response should be retried, the number of retries to perform, and how long to wait between retries
• Three new classes that implement IRetryPolicy:
  • IMDSRetryPolicy, for behavior specific to IMDS
  • ManagedIdentityRetryPolicy, for all other managed identity scenarios
  • DefaultRetryPolicy, for all other scenarios
• Minor changes to internal APIs and method calls to use the new classes

For testing:

• Much of the retry behavior remains the same and is already covered by unit tests, such as those found in RequestThrottlingTest and ManagedIdentityTests.managedIdentityTest_Retry()
• The exception is the different retry behaviors in IMDSRetryPolicy based on status codes, which is now covered by the new unit test in ManagedIdentityTests
• In addition, each retry policy class was given package-private methods to adjust the delays between retries, to significantly speed up the unit tests

Most of the "files changed" list in this PR are due to changes in two other PRs:
#960
#964

[bgavrilMS]

suggestion: All this HTTP and Retry files should go into a folder of their own?

[Avery-Dunn]

I agree that it would be good to group similar files together, but currently all of the source files are just in one big folder. I don't know why it was originally designed that way (the people who would did it are no longer here), and I've always felt it was a bad practice.

I've created an issue to track a refactor of the directory structure, at the very least there's quick value in properly organizing the non-public classes: #965