AWS SigV4 support is available in curl since 7.75.0 (December 2020).
Add the necessary bits for Hurl to understand this option and pass
it on to libcurl, both as a command line option `--aws-sigv4` and
as a per-request option `aws-sigv4` in Hurlfiles.
Do not emit `Authorization: Basic` when aws-sigv4 is used, as this
would take priority over the `Authorization` header generated by libcurl.
Instead, explicitly set `username` and `password` options.
Suppress removal of the `Expect:` header when using aws-sigv4, as a
workaround for curl/curl#11664.
Add a corresponding integration test.
