Add the Projection extension.

[lossyrob]

This adds support for the Projection extension.

This PR:

• Adds the projection extension to the pystac.extensions package
• adds the extension identifier to pystac.extensions.Extensions
• registers the projection extension in the the top level __init__.py
• Adds unit tests for the extension, including modifying the STACValidator to validate against the extension
• Adds API docs for the extension
• Documents the change in the CHANGELOG