Implement plugin-compatible architecture

Many people are asking about circle customization, too custom to be a part of the _library core_ _(e.g. arc layout, multiple values, hexagon)_.

I usually decline such PRs and suggest to use a code snippet that patches some methods (usually `drawArc` method).

But this approach is not the best - it can't be well supported, it can't be well evolved.

So the goal is to provide some plugin-compatible architecture that allows to re-use the _library core_ and to customize whatever you need.

Start by reading the library core and the existing drawArc method, which the issue identifies as a commonly patched customization point. Define how a plugin-compatible architecture would reuse the core while supporting customizations such as arc layouts, multiple values, and hexagons; completion should provide a maintainable extension path for these cases.

Written by the indexing model from the issue text.