fix: rename OPENDAL_HOT_*/COLD_* to OPENDALHOT_*/COLD_*

[aawsome]

#1392 was unfortunately ambiguous. IMO the better option to solve #1391 is to use OPENDALHOT_* and OPENDALCOLD_* env variables.

[simonsan]

Wouldn't it be more consistent to use RUSTIC_OPENDAL_* to make it less ambigious and also people can relate the env var to rustic?

[aawsome]

It is actually a bit complicated. The opendal project also uses OPENDAL_* variables in their own CLI (I think they call it oli), so it might be beneficial to also use those. Besides this, you are right that most env variables we use start with RUSTIC_* - however, these are all the ones where we have a CLI option which is replaced by this. Here we define options which are not exposed as CLI parametes, but only within the config profile..

Anyway, the amiguity if OPENDA_HOT_XXX=YYY should be interpreted as the parameter xxx=yyy for a hot repo repo (which we want here) or as hot_xxx=yyy for hot/cold part (which is what is used without this PR) must be solved, IMO.