Redefine ExpandHKDF as a one shot function

[qmuntal]

Go 1.24 will most probably move the golang.org/x/crypto/hkdf package into the standard library: golang/go#61477. The new API will be a little bit simpler, basically hkdf.Expand won't return a io.Reader but a slice of bytes as bis as the keyLength parameter.

Our cng.ExpandHKDF function is not well fitted to implement the new API, as it also returns an io.Reader and internally allocated a buffer that might not have the same size as keyLength, which will end up causing unnecessary allocations.

This PR redefines and reimplements cng.ExpandHKDF so it fits better with the new Go API. As a bonus, the resulting implementation is much simpler.

Note that this repository is still in v0, so we are free to introduce breaking changes.

[qmuntal]

For microsoft/go#1416.