Change so shared variables are utilized for AUTH

[zSleepwalker]

Right now SIG_HEADER_NAME, USER_ID_SALT, and USER_AUTH_SALT are in multiple files.
These should be moved to a shared location, so updates in the future is easier and more streamlined.

There might be more variables that could be shared between files, but these are the ones I've found.

There are also parts of the code that checks the header value, so these calls should also be updated to used the shared location.

example: var str = context.HttpContext.Request.Headers.TryGetValue("X-Red5-Signature", out StringValues header) ? header.FirstOrDefault() : null;