NIFI-6550: Create controller service for Azure Storage Credentials

[turcsanyip]

Added AzureStorageCredentialsService controller service interface
with 2 implementations:

• AzureStorageCredentialsControllerService: holds the credential properties
  (account name, account key, sas token)
• AzureStorageCredentialsControllerServiceLookup: can be used to dynamically
  look up another AzureStorageCredentialsService (similar to
  DBCPConnectionPoolLookup)

The controller service can be used by the Azure Storage processors:
{List|Fetch|Put|Delete}AzureBlobStorage, {Get|Put}AzureQueueStorage

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Enables X functionality; fixes bug NIFI-YYYY.

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[tpalfy]

LGTM +1

[ijokarumawak]

Mostly look good to me as well, but posted few minor comments to make it even better. Nice unit test coverage, thanks @turcsanyip !

[turcsanyip]

@ijokarumawak Thanks for the review! I implemented your suggestions.

[ijokarumawak]

Looks perfect! Thanks for the further improvement @turcsanyip . I'll fix the description a little bit when merging (see another comment). LGTM +1! Going to squash and merge!

[ijokarumawak]

I'll replace 'AWS' to 'Azure' when merging it :)