Add option for `tokengen` to specify the token value size during pp generation

[pasquale95]

Problem description

The public params govern the max amount a token can encode. Such information is then wrapped within the public parameters that are generated with tokengen gen....
In tokengen there is a section devoted to such task, but it seems that is not used anywhere:

fabric-token-sdk/cmd/tokengen/cobra/pp/dlog/gen.go

Lines 95 to 96 in 46bac64

	Base: Base,
	Exponent: Exponent,

.

Indeed even by specifying the --base and --exponent options, the current max value remains the same.