Cache TokenCredential

[bachuv]

This PR caches TokenCredential to resolve a memory issue that has been reported when managed identity is enabled. With these changes, we only create the TokenCredential object once instead of every time that GetStorageAccountDetails() is called. This also means that TokenRenewalState is also created once.

These changes were verified by enabling managed identity locally using these instructions and measuring the memory in VS with and without the changes in this PR.

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[davidmrdavid]

Some thoughts. In particular, curious if there's a "disposing" hook where we're supposed to remove elements from cachedTokenCredentials.