Allow for random Candid string generation

[lastmjs]

Some parameters in canister methods are just of the regular Candid string type, but the string should be a valid Candid text format string itself. Right now we don't support this, so we just generate random strings and filter out those values.

We could use didc or our own random Candid string generator to improve this.

This also brings up the question of allowing the developer to create their own custom arbitraries for specific methods and specific parameters.