Create end-to-end sample for an indexer

[heaths]

Includes AI enrichments. Fixes #12518

[heaths]

I added .gitattributes to force line endings to use LF:

countries.txt text eol=lf

All tests passed on my machine. However, on Windows agents it still got checked out to the working directory with CRLF. Upon checking this branch back out (I had switch to another branch without this file), it's checked out with CRLF again. https://www.git-scm.com/docs/gitattributes#_effects confirms this should've worked. I may instead have to implement a TextReader to normalize line endings across platforms.

[heaths]

@weshaggard is this change fine to add a line to .gitattributes? Originally I tried a file in a subdirectory but later remembered that only works for .gitignore. I need to force the line endings to LF. If not, I could write a TextReader that normalizes the line endings while the file stream is being read, but this easier to maintain.

[heaths]

I could also just "pretend" to read from a file for the sample's purpose, and in a line prepended with /*@@*/ that our sample code extractor will ignore, actually pull from a C# string literal that would be delimited by \n only. It would be nice - though not required - if we actually read from a file, though, in case customers actually look at the full sample source.

[weshaggard]

If we can eliminate the LF requirement that would be ideal but if we cannot it would be nice to keep the workaround scoped to the sample so that the sample is more portable for others that might want to consume it outside of our repo. Barring that working nicely then I'd be OK with the gitattributes change given it shouldn't impact anything else.

[heaths]

Portability is a good concern. I wouldn't expect customers to copy the full sample, but if we were to reorg the sample directory structure it would break, and figuring out why the tests all of a sudden fail wouldn't be very intuitive. I'll consider one of the alternatives.