halo-demos: Make it possible to sign raw digest using gateway_requestor.html demo

[icedevml]

Right now this demo only allows you to sign certain message in an EIP191 personal_sign format:
https://halo-demos.arx.org/examples/gateway_requestor.html

We would like to add some dropdown that would allow the user to choose whether he wants to sign the message as EIP191 message OR just sign a raw digest. In the latter case, the libhalo sign command should be invoked with the digest input key instead of message input key. See: https://github.com/arx-research/libhalo/blob/master/docs/halo-command-set.md#command-sign