[Enhacement] Extrapolate Linestring rate and distance support

[latot]

Hi all, I think would be great to can extrapolate linestrings, for this case I'm thinking when we want to extend a linestring, there is a stackoverflow with a good answer for this:

​https://gis.stackexchange.com/a/269524/213252

It proposes extend a segment where the final length is actual_length*rate_length + constant_rate. I think is a good concept.

I'll extend the concept, instead of just "extrapolate" a linestring in the end and start, we can extrapolate a segment, like ST_SetPoint, where we can extend a segment specifying the segment id (0 for first -1 for the last one). This is a way to do it, but I don't know if is the best concept for the function maybe is too general, but it is flexible.

So, here a summary I think would be fine to have before start coding.

Extrapolate the first and last segment of a linestring, or just extrapolate the "i" segment, their start/end points?

What would be the extrapolation of a multilinestring?

And a last thing, usually the easiest one is the geometry extrapolation, just the line equation, while the geography extrapolation is not trivial and does not works the method posted in stackoverflow, would be great have both of them.

Thx!