Add support for dynamic size on the ed19 opcode.

[xgreenx]

Add one more argument to the ed19 copse. Use the immediate value to set the length of the signing payload. 0 defaults to 32 as is, if a value is specified that value is followed as the payload length to sign.
Change gas metering to incorporate a dynamically signed payload, consider having a limit of 1024 for now which can be lifted later when we become comfortable with this opcode.

This changes requires corresponding modification info the specification.

[xgreenx]

Related specification PR FuelLabs/fuel-specs#600