Store the uri template string and allow it to be fetched via a getter

#### Describe the feature

Title explains it all ^^

#### Use cases

There are cases where the uri template isn't expanded right away, and the original template string needs to be accessed. My particular use case involves an API that rate limits requests based on its URI template, so I need to access the original string to properly handle rate limiting. For now, I am simply going to pass the string separately, but it makes sense that a URI template object would have some way to access its template.

#### Contribution

Willing to PR for this feature if required.