Introduced an `ApiKeyHttpMessageHandler` to use an API key as the authorization header for Elsa API client. This ensures secure interaction with the Elsa server. Also made updates to the `ElsaClientOptions` class to include the API key. An `AddElsaClient` method is added with the API key parameter in `DependencyInjectionExtensions` class for convenience.
The ApiHttpMessageHandler and the related configuration in ElsaClientOptions have been removed. The configuration for creating Api clients in DependencyInjectionExtensions has been updated to handle these changes. This includes the addition of a retry policy in case of transient HTTP error and changing the way clients are created to use the HttpClientFactory directly.
