fix(core): make randomString deterministic and allow larger values

[bernardobridge]

Description

There are two issues with the current $randomString function:

• It will sometimes return a different length than specified, due to trailing zeroes
• It actually doesn't work for large lengths (n>10 I believe). It just always returns up to 10 chars

I tried and benchmarked quite a few possible variations of the function (e.g. using crypto.randomBytes, lodash utilities, etc). This solution was by far the fastest I got, even surpassing the existing function.

Notes:

• This solution allows capital letters, but we can remove that if we want (I believe the previous solution didn't).
• Defaults to length=10 if no length is specified to keep it backwards compatible, since the previous function implicitly did this

Testing

• Added thorough unit test that checks for the regression. I also ran it with the old function, and it catches the issue (i.e. fails successfully)
• Benchmarks to make sure it works at load-test scale

Pre-merge checklist

• Does this require an update to the docs? These functions aren't mentioned in the docs. I think maybe we should add $randomString and $randomNumber to the docs as part of this
• Does this require a changelog entry? Yes