resolver: Add new fields to resolver.BuildOption struct to support dialing a remote name resolver

[easwars]

These fields will be used by resolver implementations which need to talk
to a remote name resolver.

[easwars]

@dfawley
Based on our offline discussion, I also added the CredsBundle field to the BuildOption struct.

[dfawley]

Can you update this PR so that it also sets all the fields? Or was this PR intended to be a proposal / strawman for discussion purposes?

[easwars]

Can you update this PR so that it also sets all the fields? Or was this PR intended to be a proposal / strawman for discussion purposes?

I was initially being lazy and thought I'd do that with the xdsResolver changes. But makes sense to do it here. Done.

[easwars]

Ping ...

[dfawley]

Maybe this should say simply that it's the DialCreds for the ClientConn? It may not be appropriate to use these credentials for your name resolver in many (practically all?) cases. Same for the other two fields, except that a Bundle is a little more likely to be OK to use. Possibly include a warning about using this.

[easwars]

Does this sound OK? If so, I can make similar modifications to the other two fields as well.

DialCreds is the transport credentials used by the ClientConn (set as a DialOption
using a call to WithTransportCredentials) while communicating with the gRPC
server. In some cases, it may be appropriate for a resolver implementation to use 
these credentials to talk to a remote name resolution server. In most cases though, 
it might not be appropriate and should be used with caution.

[dfawley]

How about:

DialCreds is the transport credentials used by the ClientConn for communicating with the target gRPC service (set via WithTransportCredentials). In cases where a name resolution service requires the same credentials, the resolver may use this field. In most cases though, it is not appropriate, and this field may be ignored.

[easwars]

How about:

DialCreds is the transport credentials used by the ClientConn for communicating with the target gRPC service (set via WithTransportCredentials). In cases where a name resolution service requires the same credentials, the resolver may use this field. In most cases though, it is not appropriate, and this field may be ignored.

Done. Thanks.