Request extension files and libraries from compute_ctl v14

[lubennikovaav]

postres part of neondatabase/neon#4592

[hlinnaka]

Would it be possible to implement this with DDL event triggers or as a ProcessUtility hook?

[lubennikovaav]

Would it be possible to implement this with DDL event triggers or as a ProcessUtility hook?

Well, I think it is not absolutely impossible, but why?

This approach will require to repeat most of these functions:

• First, we'll need to parse control file to figure out extension directory (if any), library name and path and so on.
• Second, we'll need to check if files are present on disk.
• Then request them from remote extension server.
• And finally try to read file locally again.

So, I decided to add small patch to existing code instead.
All changes are inside in if (download_extension_file_hook != NULL) to ensure that it doesn't affect normal execution.