Make signature popup position configurable?

I like the popup call signatures, but their positioning isn't great. They appear directly above the cursor, usually covering the last few lines of code context.

I'd like the popup to spawn somewhere else, preferably with a way to customize its position. Possibilities:

* Below the current line instead of above it
* A line removed from the current one, so there's more immediate context
* At the top or bottom of the current split
* In a tiny hsplit, maybe? I gather from other issues that the in-buffer popup feature is a bit gnarly, so possibly this is harder to change than it sounds.

For the time being I'm using `show_call_signatures=2`, which works fine, although I sort of wish it aligned the message with split column 0 instead of the cursor position, and it's harder to read alternate signatures.

I'm not great at vimscript, but I'm willing to attempt to implement this if someone can point me to the right piece of code.