Add download_has_headers_param feature (#30)

Coming in 7.1.0, rctx.download will accept a headers parameter allowing
arbitrary headers to be passed to the downloaders.
Link to issue: https://github.com/bazelbuild/bazel/issues/17829

cc: @fmeum

features.bzl

diff --git a/features.bzl b/features.bzl
index dffed76..b314d17 100644
--- a/features.bzl
+++ b/features.bzl
@@ -24,7 +24,9 @@
     # 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")
+    download_has_block_param = ge("7.1.0"),
+    # Whether repository_ctx#download has the headers parameter, allowing arbitrary headers (#17829)
+    download_has_headers_param = ge("7.1.0")
 )
 
 _flags = struct(