Middleware user metadata replacer

Some object stores like Azure support a limited set of user metadata names and values, e.g., no hyphens. This middleware allows replacing the characters during putBlob and getBlob so that the client can interact with objects without code changes. Configuration:

s3proxy.user-metadata-replacer-blobstore=true
s3proxy.user-metadata-replacer-blobstore.from-chars=_
s3proxy.user-metadata-replacer-blobstore.to-chars=-