Search API Integration: Add Search API Processor Plugin to filter by affiliate domain

Adding related issues links:

• #2186437: Domain Access + Views + Search API →
• #1508526: Content Viewing and Domain Access →
• #2302759: Filter Domain Access in Search API view →
• #1969938: Integration with search module →

Here's the patch file also.

I was unsure about which module would include this; but went with domain_content.

I was also unsure if the search_api should be listed as a module dependency. I omitted it for now. I imagine that the domain module maintainers
may not want this functionality included within the project, and this should be a separate module. Feedback welcome either way.

I have yet to fully verify this works as intended. If it doesn't work. I'll update the issue.

Adding another related issue from Metatag module, ✨ Add new Search API processor plugin for more generic data indexing Needs work , which represents a different use case.

jasonawant → closed merge request !76

The latest MR and this patch fix a logic error and now provides support for Send to all affiliates option.

Feedback welcomed!