Allow http GET method for introspection query

[ecgreb]

Use case

Hi Apollo team, thanks for all the great work on this project! Are there plans to support http GET method for introspection queries? Some CMS platforms (ex. Prismic) require GET to be used for all queries including introspection.

Describe the solution you'd like

Allow setting of http method via introspection block in gradle config.

build.gradle.kts

apollo {
    service("prismic") {
        packageName.set("com.example.android")
        introspection {
            endpointUrl.set("https://example.prismic.io/graphql")
            schemaFile.set(file("src/main/graphql/schema.graphqls"))
            httpMethod.set("GET") // Proposed solution
        }
    }
}

[martinbonnin]

This must be quite the GET url. Introspection are not particularly "short" 😄 . But adding a httpMethod param sounds good to me.

Do you want to open a PR?