Handle events that are part of client API surface

Currently, events make a bunch of our analyzers to fire:

```
#pragma warning disable AZC0002 // DO ensure all service methods, both asynchronous and synchronous, take an optional CancellationToken parameter called cancellationToken.
#pragma warning disable AZC0003 // DO make service methods virtual.
#pragma warning disable AZC0004 // DO provide both asynchronous and synchronous variants for all service methods.
#pragma warning disable AZC0015 // Unexpected client method return type.
```

Start by locating the analyzers that emit AZC0002, AZC0003, AZC0004, and AZC0015, then inspect how event members are classified as part of the client API surface. Done means events no longer cause inappropriate service-method warnings while the relevant analyzer behavior remains covered.

Written by the indexing model from the issue text.