node: Add ability to pass custom headers to Sui gRPC

[djb15]

No description provided.

[johnsaigle]

suggestion: I think some of this logic could be replaced by using cobra's built-in parsing.

This is an example that parses key-value pairs:

var labels map[string]string

cmd.Flags().StringToStringVar(
    &labels,
    "label",
    nil,
    "key-value labels, e.g. --label env=prod,team=infra",
)

It might make more sense to put this in a the cmd/ code as well since this is related to CLI input parsing rather than Sui client code.