Ability to generate json schema from a local file via the downloadApolloSchema gradle task.

Our Android apollo setup right now downloads a GraphQL SDL file manually (It's a custom POST endpoint).

We manually invoke apollo CLI's client:downloadSchema with a relatively basic service config

`module.exports = {
client: {
service: {
name: 'myservice',
localSchemaFile: '../../schema.graphql'
},
includes: ['**/*.graphql']
}
}`

This is great, and allows us to generate the JSON schema off of a local GraphQL SDL file rather than hitting an endpoint.

I do not seem to be able to replicate this functionality through the downloadApolloSchema gradle task. While this task seems to just wrap client:downloadSchema, it appears to have the additional requirement of endpoint being set.

My feature request is to have the ability to generate the GraphQL schema JSON off of a local SDL file via the gradle plugin, mirroring the functionality of the raw CLI interface.

I would be willing to work on an implementation of this if someone could point me in the direction of where the gradle task is created/exposed.

Start by locating the downloadApolloSchema Gradle task and its wrapper around client:downloadSchema; compare its endpoint requirement with the CLI's localSchemaFile configuration. Done means the Gradle task can generate a JSON schema from a local GraphQL SDL file without requiring an endpoint.

Written by the indexing model from the issue text.