Multiples services for one client

Ability for emitters to generate one client from multiple services defined in TypeSpec.
Useful for ARM services like Compute where multiple services (VMs, Disks, Networks, etc.) are defined but a single client is expected.

### Spec

### Implementation

The issue names no files, tests, or entry points. Start by tracing the emitters and how TypeSpec services are defined, then clarify the expected single-client behavior for ARM-style services such as Compute before defining tests for multiple services.

Written by the indexing model from the issue text.