added single request feature

[sentd94]

This PR adds a new method, singleWithin, that allows users to retrieve documents near a given point as a single Future, instead of a Stream. This feature is particularly useful for users who only need to access the data once, as it simplifies the process and reduces overhead.

[sentd94]

Fixes #6

[kosukesaigusa]

I also like the idea of enabling the package to fetch geo locations once as Future!

And actually, the package below, heavily inspired by geoflutterfire/geoflutterfire2, is released a few weeks ago with full respect!

https://pub.dev/packages/geoflutterfire_plus

https://github.com/KosukeSaigusa/geoflutterfire_plus

• Almost all the existing features are available.
• But whole codes are redesigned and refactored for future extensibility and easy maintenance.
• Some of the definitions and names of classes, methods, and variables are different, so once you may need to refactor you code🙏
• The fetching once as Future method is already available.
• Some other new features such as custom query are available, and unit tests are being added aiming for 100% coverage.

If you are interested, please check geoflutterfire_plus!

Of course, feature/pull requests are always welcome🙌

Sorry for the comments from outside!