exchange: parse requirements from stream clone specification string Stream clone bundles can only be consumed if the consumer supports the exact format requirements that were present on the producer. This patch adds support for encoding and verifying the format requirements on the bundle specification string for a stream clone bundle are supported by the local repository. If they aren't, we raise an UnsupportedBundleSpecification, just like we do when an unknown compression or bundle type is encountered. The impetus for this patch is so the clone bundles manifest can advertise stream clone bundles and so clients can filter out stream clones with unsupported format requirements. e.g. a stream clone produced with the not-yet-invented "revlogv2" format will be ignored by clients that only support "revlogv1."