feat(utils): add random module

[elenaf9]

Description of change

Extract random methods into a single module stronghold_utils::random.
Ensure that random::bytestring() and random::string() are never empty.

Type of change

• Enhancement (a non-breaking change which adds functionality)

Change checklist

Add an x to the boxes that are relevant to your changes, and delete any items that are not.

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes

[felsweg-iota]

Better move magic constant outside, or make the function call configurable in terms of range.

[felsweg-iota]

looks good to me