exchange: support parameters in bundle specification strings Sometimes a basic type string is not sufficient for representing the contents of a bundle. Take bundle2 for example: future bundle2 files may contain parts that today's bundle2 parser can't read. Another example is stream clone data. These require clients to support specific repository formats or they won't be able to read the written files. In both scenarios, we need to describe additional metadata beyond the outer container type. Furthermore, this metadata behaves more like an unordered set, so an order-based declaration format (such as static strings) is not sufficient. We introduce support for "parameters" into the bundle specification string. These are essentially key-value pairs that can be used to encode additional metadata about the bundle. Semicolons are used as the delimiter partially to increase similarity to MIME parameter values (see RFC 2231) and because they are relatively safe from the command line (although values will need quotes to avoid interpretation as multiple shell commands). Alternatives considered were spaces (a bit annoying to encode) and '&' (similar to URL query strings) (which will do bad things in a shell if unquoted). The parsing function now returns a dict of parsed parameters and consumers have been updated accordingly.