[filebeat][azure-blob-storage] Support custom content-type

[strawgate]

Describe the enhancement:
The S3 input has a content-type setting which allows overriding the content type in the case that the user can assert the blob is of a specific type. We should add the same feature to the azure-blob-storage input.

Describe a specific use case for the enhancement or feature:
The content type of an object is decided by the uploader, when we download the file we only allow specific content types and so if the uploader has incorrectly set the content type, we fail to process the file.

[botelastic]

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