Extend fuel-client queries to return the "extensions" map from responses

[acerone85]

Following #2473, the FuelClient can setup the current_fuel_block_height extensions field in requests as follows:

let mut client: FuelClient = FuelClient::from(srv.bound_address);
client.with_required_fuel_block_height(100);

However, currently there is no way to retrieve the response headers from the client. There are two main issues that prevent it:

• the Client::query method returns an Err value if queries fail, but we want to return the extensions map also in case of errors.
• changing the signature of Client::query would break several tests

Proposed solutions:

1. Add a field to store the response extensions to the Client
   Pros:

• does not require changing the signature of Client::query.
• Can retrieve the extensions map in case of errors
  Cons:

• Would not work very well with parallel tests

2. Create a new Client::extended_query that returns the extensions map together with the graphql response
   Pros:

• Each requests will receive its own extensions map. Works well for parallel tests

Cons:

• Would still not be able to retrieve the map in case of errors
• Each of the function that calls query (there are quite a lot) will need a counterpart that calls extended_query (we could have a macro for that)

3. Create a new Client::extended_query function that takes in input a mutable reference to the extensions map to be filled.

Pros:

• Each request will have its own extension map. Works well for parallel tests
• Will still be able to set the extensions map in case of error

Cons:

• Setting a mutable map is kind of a side effect
• Will still need to create an "extended" counterpart to each function that calls "Client::query".