Add support BasicInterpolation from python configparser

[stalkerg]

In the python version, we have special variables like %(here) which is very helpful for big config files, but mostly it's needed for interoperability.
https://docs.python.org/3/library/configparser.html#configparser.BasicInterpolation

[QEDK]

In the python version, we have special variables like %(here) which is very helpful for big config files, but mostly it's needed for interoperability. https://docs.python.org/3/library/configparser.html#configparser.BasicInterpolation

I see two ways to go about it, one is getter-level interpolation which is fairly easy but adds more functions, get_with_interpolation() or similar. Second is struct-level interpolation which is more useful but more complicated, similar to defaults, specify a interpolation hashmap like HashMap(String, String) and replace values at the time of parsing.

[stalkerg]

It seems like even ExtendedInterpolation is about interpolation after parsing for each value. I think get_with_interpolation can be good enough.
From a performance point, you probably want to see both ways because sometimes significant the config parsing time and sometimes how quickly you can get data. But if you can get value, it's not a big deal to cache it.

[belak]

What sort of API are you hoping for? We ended up implementing it on top of the library, but it would be nice to upstream it if possible.

It does introduce new failure modes though: for our use case, we ended up ignoring invalid interpolation and just passing through the value directly, but I have to imagine some people would prefer to receive that error.