Add Either type for EventGrid output connections

[castrodd]

In PR #178 we faced the problem that there are two exclusive ways to configure an EventGrid output binding: key-based or identity-based.

However, TypeScript does not have a built-in utility type that forces users to implement one interface or another but not both. We left the code in a state where users would be warned if they:

1. Used topicEndpointUri or topicKeySetting but not both
2. Did not specify any properties

But users were free to combine the connection property with topicEndpointUri or topicKeySetting or both without any warnings or errors thrown by the type checker. Our only defense against this was a comment that When setting the connection property, the topicEndpointUri and topicKeySetting properties should NOT be set.

Ideally, this comment would be unnecessary and our type design would prevent any such misconfigurations.

This PR introduces a custom utility type Either. It takes two interfaces A and B and allows two possible implementations: all the properties in A are accepted but none of the properties in B or vice-versa.

Now if a user tries to supply a connection property along with key-based settings, this usage will be flagged:

Type '{ topicEndpointUri: string; topicKeySetting: string; connection: string; }' is not assignable to type '{ topicEndpointUri?: never; topicKeySetting?: never; }'.

Type '{ topicEndpointUri: string; connection: string; }' is not assignable to type '{ topicEndpointUri?: never; topicKeySetting?: never; }'.

Type '{ topicKeySetting: string; connection: string; }' is not assignable to type '{ topicEndpointUri?: never; topicKeySetting?: never; }'.

And the correct implementations of both EventGridOutputKeyOptions and EventGridOutputConnectionOptions still work as expected: