[IDP-1715] Improve docs to show how to modify Event Grid client options and expose the client name

[asimmon]

Description of changes

• Add doc to show how to configure EventGridPublisherClientOptions and EventGridClientOptions.
• Changed the internal client names (previously EventPropagationClient and NamespaceTopicClient) to a public, single name DomainEventPropagation
  • I search for usage of these strings in our codebases => no results
  • It's ok to reuse the same string. .NET named options work per option type AND option name. We already did that in another lib (auditing).

Breaking changes

• The Event Grid client names have changed but nobody was refering to them
• The client name is now publicly exposed for people to override the options

Additional checks

• Updated the documentation of the project to reflect the changes
• Added new Modified tests that cover the code changes