Add has_parallel_download feature to detect if rctx.download supports parallel downloads

[mortenmj]

Coming in 7.1.0, repository_ctx#download has a new argument, block:

If set to false, the call returns immediately and instead of the regular return value, it returns a token with one single method, wait(), which blocks until the download is finished and returns the usual return value or throws as usual.

It would be useful to be able to update repository rules to use this feature when available.