Customise merge behaviour depending on option. Fix #1803 (#1984)

Fix for #1803, where the method used for merging the config-settings option - string concatenation - was of little use.

I broke out the TableFmt object into a protocol-like object called OptionFormat, where the formatting and merge rules can be customised more deeply. This is more explicit, and lets us be a bit more precise with how TOML object get converted into the options.

I intend to use the flexibility of the OptionsFormat structure to tackle the remaining abiguity around environment variable quoting, as seen in the xfails in the options_test.py unit test and discussed in #1271.


Co-authored-by: Matthieu Darbois <mayeut@users.noreply.github.com>