Change GetDaxConn to Dax's method if type parameters are allowed in methods

[sttk]

Since golang does not allow type parameters on methods now, GetDaxConn[D DaxConn] is defined as a global function.
If the following proposal is adopted, I'll change GetDaxConn[D DaxConn] to Dax's method.

proposal: https://github.com/golang/go/issues/49085

Example:

    conn, err := sabi.GetDaxConn[MyDax](dax, "database");
        ↓
    conn, err := dax.GetDaxConn[MyDax]("database");