Make SRandom::randStr() not require a reference

[coleaeason]

@tylerkaraszewski This updates SRandom::randStr() to not require a reference. I'm not sure why we originally implemented it this way (we being me). We have a bunch of code in Auth like this:

int64_t size = 123;
string rand = SRandom::randStr(size);

instead of being able to just do this:

string rand = SRandom::randStr(123):

Fixed Issues

Confusing code

Tests

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Internal Testing Reminder: when changing bedrock, please compile auth against your new changes