Rename "DelegateEndpoint" types

[halter73]

Given that the options type related to the Map(string, Delegate)-style methods is named RouteHandlerOptions, we should make the following more consistent.

• DelegateEndpointConventionBuilder -> RouteHandlerBuilder
• OpenApiDelegateEndpointConventionBuilderExtensions -> OpenApiRouteHandlerBuilderExtensions
• DelegateEndpointRouteBuilderExtensions merges with the existing EndpointRouteBuilderExtensions.

Basically, this changes DelegateEndpoint to RouteHandler and removes Convention in the class names.

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