sagemaker: Make IEndpointConfig property of Endpoint optional

### Describe the feature

Even though explicitly defining an `EndpointConfig` resource allows for reusable configuration across endpoints, as [discussed on the original `Endpoint` L2 construct PR](https://github.com/aws/aws-cdk/pull/22886#discussion_r1028488567), customers looking to deploy a single endpoint may find it easier to have the endpoint configuration encapsulated within the endpoint construct.

**Please 👍 this issue to help with the prioritization of this feature.**

### Use Case

Customers may not want to have to define their own `IEndpointConfig` resource, instead relying on `Endpoint` to instantiate one for them (in the event they don't provide one on `EndpointProps`).

### Proposed Solution

Add each of [these members of `EndpointConfigProps`](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/aws-sagemaker/lib/endpoint-config.ts#L132-L145) to `EndpointProps` as optional arguments (while making the existing `IEndpointConfig` prop value optional as well) while ensuring that the `IEndpointConfig` property and the new members are mutually exclusive.

In this scenario, it likely also make sense to include an `addInstanceProductionVariant` on `Endpoint` so that customers can mutate the backing `EndpointConfig` in the same manner as they could using their own instance (this does have the added bonus of returning an `IEndpointInstanceProductionVariant` instance from the method, eliminating the need for customers to look up the variant by name).

### Other Information

_No response_

### Acknowledgements

- [X] I _may_ be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.54.0-alpha.0

### Environment details (OS name and version, etc.)

macOS Ventura

Start with the EndpointProps and Endpoint implementation, then read the EndpointConfigProps members in packages/@aws-cdk/aws-sagemaker/lib/endpoint-config.ts. Define the supported mutually exclusive inputs and consider how Endpoint would expose addInstanceProductionVariant; done means single-endpoint users can omit IEndpointConfig while existing explicit configurations remain supported.

Written by the indexing model from the issue text.