Support default timeouts in clients

[timostamm]

Is your feature request related to a problem? Please describe.
In a client, a timeout for an RPC can be configured per call:

// the call will be cancelled after 3 seconds
client.introduce({ name }, { timeoutMs: 3000 })

In a setting where many different streaming procedures are called, it is not a good developer experience having to remember to set a timeout on every call.

Describe the solution you'd like
It would be a convenient to be able to specify a default timeout on the transport instead, for example:

createConnectTransport({
  baseUrl: "https://demo.connectrpc.com",
  defaultTimeoutMs: 3000, // applies to all calls that do not specify a timeout
})

If an exemption needs to be made, the default timeout could be overridden by passing timeoutMs: undefined (or perhaps timeoutMs: null to be more explicit) to a call.