Add docker_context segment

[treuherz]

Adds docker_context segment to show current docker context, with
SHOW_ON_COMMAND and SHOW_DEFAULT parameters.

I hope submitting a PR for this is ok. I would have commented on the issue first but by the time I'd had a go to see if it seemed doable I was most of the way done!

I'm sure there are ways this could be more performant, any advice would be appreciated. docker context show is slow, ~100ms, so this is much faster when jq is available. I don't have a windows machine available to test whether it can find the config file ok on there, so that might need work.

Closes #1485

[romkatv]

Thanks for submitting a PR.

This code is too slow for inclusion in powerlevel10k even when jq is available. If you want to use this segment, put it in your own config, rename prompt_docker_context to prompt_my_docker_context (to avoid clashes with future versions of p10k) and replace _p9k_prompt_segment with a call to p10k segment (see p10k help segment).

[treuherz]

Understood, I appreciate you need to maintain high performance standards in here.

Even if it's not going to be merged, pointers on how to improve it would be great, but no worries if you don't have the bandwidth 👍

[romkatv]

I don't have the bandwidth. It'll take me less time to implement this segment than to explain how to implement it.