Replace macos and windows client implementations with a generic one

[sorindumitru]

It looks like the client here is only supported on Linux. There's two implementations for 2 OSs, I assume to prevent build failures when using the library from those systems. I'd like to be able to do the same for FreeBSD. Instead of adding a client_freebsd.go, it might be best to just have an implementation for non-linux OSs.

This seems to build ok (I don't actually have a FreeBSD system available):

# Before
% GOOS=freebsd go test ./client/
# github.com/google/go-tdx-guest/client [github.com/google/go-tdx-guest/client.test]
client/client.go:147:17: undefined: OpenDevice
client/client_test.go:60:17: undefined: OpenDevice
FAIL    github.com/google/go-tdx-guest/client [build failed]
FAIL
# Now
% GOOS=freebsd go test ./client/
signal: segmentation fault (core dumped)
FAIL    github.com/google/go-tdx-guest/client   0.054s
FAIL

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