x/crypto/bcrypt: add cooperative scheduling and cancellation to bcrypt

[hf]

Password hashing functions like bcrypt are very CPU intensive and long running. On my Apple m1 chip, it takes about 75,455 microseconds for one password hash to be generated with the current DefaultCost of 10. Furthermore, the current algorithm is not cooperative with the rest of the Goroutines in the system, which can relatively easily create latency issues on other less CPU intensive Goroutines. This is in-particular a major issue in web servers implementing sign-in / sign-up with password.

This PR adds CompareHashAndPasswordWithContext and GenerateHashWithContext functions that call into runtime.Gosched() every 64 bcrypt rounds (i.e. about every 4.7 ms on m1). These also allow obeying a cancellation signal from the context. The existing functions are intentionally not cooperative.

(I'm open to tweaking the number of rounds.)

Fixes #56416

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Message from Roland Shoemaker:

Patch Set 4: Hold+1

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Patch Set 5:

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Message from Roland Shoemaker:

Patch Set 4: Hold+1

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