add DialOptions to GRPCBroker's Dial

[alexandreLamarre]

Motivation

Spiritual successor to : #168

For purpose built client-side interceptors, there can be a need to control which plugins get certain interceptors and this currently isn't possible with GRPCBroker's Dial(id uint32) signature or using "global" dialOpts in the plugin Client Config struct (without changing existing plugin structures to use multiple client instances)

Client-side change

The underlying dial implementation in Dial(id uint32) accepts ...grpc.DialOption so I propose :

Dial(id uint32, dialOpts ...grpc.DialOption) for more fine grained control over GRPCBroker connections.

[hashicorp-cla]

All committers have signed the CLA.

[robmonte]

Closed by #257