Add middleware to replace ETag

[gaul]

PutObject responses from AWS include a base64-encoded ETag which other requests like conditional GetObject use. This ETag is the MD5 hash of the object which some clients will compare against the client-generated MD5. Azure and B2 return a non-MD5 ETag which makes those clients fail their PutObject requests. Could S3Proxy add a middleware to calculate the MD5 and return it?