Add ability to configure minimum idle connection

[leotcsun]

Currently it is not possible to configure the minimum number of connection for the client. Can this ability be added to the SDK so that we can reuse idle connections and avoid the cost of establishing new connections (especially when TLS is enabled and new connections are more expensive to make).

[nemorales]

Would be amazing to have this implemented, as in our use case we see situations in which a newly created client cannot handle multiple calls at the same time