Add has_parallel_download feature to detect if rctx.download supports…

… parallel downloads (#29)

Coming in 7.1.0,
[repository_ctx#download](https://bazel.build/rules/lib/builtins/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.

features.bzl

@@ -23,6 +23,8 @@ _external_deps = struct(
     # Whether module_extension has the os_dependent and arch_dependent parameters.
     # https://github.com/bazelbuild/bazel/commit/970b9dda7cd215a29d73a53871500bc4e2dc6142
     module_extension_has_os_arch_dependent = ge("6.4.0"),
+    # Whether repository_ctx#download has the block parameter, allowing parallel downloads (#19674)
+    download_has_block_param = ge("7.1.0")
 )
 
 _flags = struct(