[http] Allow issuing requests with an explicitly specified HTTP method

The design of IMDSv2 within both AWS and Alibaba Cloud requires the
client to obtain a temporary token via an HTTP PUT request.  There is
no authentication on this request and there is no associated request
body: the requirement to use PUT exists solely to reduce the attack
surface for SSRF attacks (since vulnerable servers are much more
likely to be able to be tricked into issuing a GET request than a PUT
request).

iPXE can currently issue requests using HTTP GET (if the request body
is empty) or HTTP POST (if the request body includes form parameters).
There is no support for issuing a PUT request, or for allowing a
script to explicitly specify the HTTP method.

Add a "--method" option to the "params" command to allow an arbitrary
request method name to be specified, and use this as the HTTP request
method.

Signed-off-by: Michael Brown <mcb30@ipxe.org>