Add support for "starts-with" in user-metadata post policy condition.

[hsequeda]

Currently, it only supports the "Exact match" but AWS supports also starts-with in this case (check doc => https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html). Also, I want to be able to allow some metadata fields but without limit it, so will be great if emptyValue restriction of SetMetadata is not there any more (for the starts-with case).