This library should allow to specify a proxy without env vars

[LsKoder]

The unique way we have to use a proxy for PubSub Client is to set https_proxy over env variable.
This is a limit for this library because it could generate a conflict with other libraries that use the same logic.
We should be able to specify the proxy only for a specific service (like google), not globally.

A way to set https proxy should be allowed directly into constructor PubSub().
Please add a proxy option into ClientConfig configuration.
Eg.

export interface ClientConfig extends gax.GrpcClientOptions {
    apiEndpoint?: string;
    servicePath?: string;
    port?: string | number;
    sslCreds?: gax.grpc.ChannelCredentials;
    proxyHttps?: string; // new parameter suggested
}

Thanks