isSubnameAvailable to check if a subname can be registered.
Parameters
subname: Subname to check availability for (e.g.,"ns.myensname.eth")
Return Type
isAvailable:trueif the subname is available,falseif already taken.
Always check subname availability before attempting to create or update a subname.
It may overwrite the existing subname.
It may overwrite the existing subname.

