Retrieve MD5 from object metadata

[joneubank]

For S3 downloads, when handling downloads in cases where the .meta file is not found, we need to retrieve the MD5 sum from the Object metadata. Our current implementation of this is reading the value of the eTag, but this is not guaranteed to contain an MD5 hash (and for multipart uploads is known to never contain the MD5 hash).

This change introduces a function to get the MD5 hash from the metadata. The function has two cases to check for where the MD5 hash might be.

1. Priority is to use the built in S3 getContentMD5() method. If the hash is found here then this will be returned.
2. Allow reading of a custom (user defined) metadata property. This allows a system administrator to assign the Base64 encoded MD5 hash in a custom location. This is given the default property name md5chksum which corresponds to the property name where the md5 is stored in Azure systems. This will match for cases where data is moved from Azure Blob storage into an S3 Object Storage system.

[Azher2Ali]

LGTM!