Consider `application` extension for processing script reference

[fmigneault]

🚀 Feature Request

At the moment, the processing extension is recommended to describe reference scripts
(https://github.com/stac-extensions/mlm/blob/main/best-practices.md#processing-extension).

Another emerging application extension is proposed to contain script references and details about them being embedded within another type (eg: markdown containing python code).

This should be added to best practices examples.

https://github.com/stac-extensions/application

🔉 Motivation

Alignment and comprehension of definitions by users.

📡 Alternatives

• Provide examples of processing extension #116