Add Support for Custom Root Operation Type Names in GraphQL

[chathuranga-jayanath-99]

Description: GraphQL schema can be represented in following 2 methods,

Method 1

type Query {
    someField: String
}

Method 2

schema {
    query: MyQueryRootType
}

type MyQueryRootType {
    someField: String
}

Method 2 is need to be considered when sending introspection queries. Currently the implementation doesn't check this.

[ThisaruGuruge]

In the second approach, the root query operation type has a different name. The only difference here is when introspecting, the type name would be different. This can be achieved if we allow to define separate service types for root operations. Might be related to #1382 and more precisely, providing a way to define GraphQL schema/service separately from the HTTP listener.

[ThisaruGuruge]

Related to #4620