Create Site Search implementation

[unclecheese]

As a Silverstripe CMS developer, I want my Silverstripe CMS project to index
selected content types into the Elastic Site search SaaS product so that I can create an excellent search experience for the end users of my project.

Acceptance criteria

• Zero-config opt-in (e.g. with environment variable change)
• Declare fields for faceting in config
• Set attributes for faceting in config
• Declare searchable types in config
• Declare implementation-specific options in config

Excludes

• Crawler support
• Querying the Site Search API