Reduce cost of CreateSecKeyAndSecWebSocketAccept (#49831)

Reduce cost of CreateSecKeyAndSecWebSocketAccept (#49831)

This isn't a big deal as it's once per websocket connection, but it annoys me every time I see the unnecessary waste in a trace.  Previously this would allocate:
1. A byte[] for the bytes from the guid
2. A base64-encoded string for (1)
3. A string concatenating (2) and the known server guid
4. A byte[] containing the bytes from (3)
5. A byte[] containing the SHA1-hashed bytes of (4)
6. A base64-encoded string of (5)

Now, it only allocates (2) and (6).