Support multiple schema sources

Currently, the VS Code Apollo extension and Apollo CLI support defining only a single schema/service in the `apollo.config.json` file.

In projects that consume multiple GraphQL APIs (for example, `main-api` and `analytics-api`), we need to maintain separate config files like:
- `apollo.config.main.json`
- `apollo.config.analytics.json`

It would be helpful to support multiple schema sources or clients within a single config file, something like:

```json
{
"clients": [
{
"includes": [
"./src/apollo/fragments/**",
"./src/apollo/queries/**/*.{gql,graphql}",
"./src/apollo/mutations/**/*.{gql,graphql}"
],
"name": "main-api",
"service": { "url": "https://example.com/graphql" }
},
{
"includes": [
"./src/apollo/analytics/fragments/**",
"./src/apollo/analytics/queries/**/*.{gql,graphql}",
"./src/apollo/analytics/mutations/**/*.{gql,graphql}"
],
"name": "analytics-api",
"service": { "url": "https://analytics.example.com/graphql" }
}
]
}

Start by tracing how the VS Code extension and Apollo CLI read and validate apollo.config.json, focusing on their current single-schema or single-service assumptions. Compare the proposed clients structure with existing configuration behavior, then define tests for two named clients with separate includes and service URLs; done means both tools can discover and process multiple schema sources from one config file.

Written by the indexing model from the issue text.