New arguments to from_schema() to constrain generated strings

[Zac-HD]

In principle, implementations ought to handle any representable data. In practice... practicality sometimes beats purity. I therefore propose that we should add two new arguments to from_schema(), and plumb them through.

allow_x00: bool = True

Requests containing the zero codepoint, "\x00", are rejected as invalid by many servers (see e.g. schemathesis/schemathesis#1286, schemathesis/schemathesis#1562, schemathesis/schemathesis#1668). Arguably they should, but in the meantime it seems useful to offer Schemathesis a mechanism to exclude the zero codepoint and schema-level granularity seems sufficient.

This is a fairly simple matter of passing through the argument, and then adding min_codepoint=1 if it's False. I'm unsure what we should name it; leading candidates are allow_x00 to emphasize the low-level nature or allow_codepoint_zero to spell it out.

codec: str | None ="utf-8"

Per schemathesis/schemathesis#1142 (comment) and schemathesis/schemathesis#1286, users often want to restrict inputs to the ASCII range. This is fundamentally kinda wrong, but we live in a broken world and sometimes harm reduction (i.e. allowing people to use advanced testing tools anyway) is the best option left to us.

Implementation here is easy in hypothesis-jsonschema, but requires upstream issues as a prerequisite: HypothesisWorks/hypothesis#1664 and HypothesisWorks/hypothesis#3479.