Make spring-cloud-bindings API extensible & reusable

[jxblum]

As it stands currently, this project offers a nice API, but is not quite a Framework that others can easily reuse and extend.

This stems from 2 noticeable limitations:

1. Both the Guards and MapMapper types are package-private, making them difficult to reuse in external implementations of the BindingsPropertiesProcessor interface for convenience.

2. The Binding and Bindings classes really should not be final as it makes them difficult to mock, or at least spy on in a true Unit Test.

Making these changes will make the project more Framework-like, being 1) reusable and 2) "open for extension, but closed for modification".

Thank you for the consideration.

[jxblum]

For one such example of an extension outside the spring-cloud-bindings project itself, see here.

[dmikusa]

Hi @jxblum I don't have any objection to the change, just curious as to why you would not instead consider submitting a PR to include your extension bindings here? That way they are all in one place.

Alternatively, we'd need to maintain a list of known external extensions so that users can easily discover what else exists, outside of this project. It's not insurmountable, we could list them in the README.

Just curious your rationale for wanting to maintain the bindings elsewhere. Thanks

[dmikusa]

See for additional context, spring-projects/spring-boot-data-geode#95.