sigRequestHash not a descriptive param

[frankiebee]

When reviewing the signing message with @mix in reference to the validation milestone we found this param name to be confusing an non descriptive.
This is suposed to be a hex encoded string of the data to be signed

sdk/src/signing/index.ts

Line 192 in ce96745

sigRequestHash,

the proposal is to have:

a) better docs around this
b) rename sigRequestHash -> hexMessage