introduce context show command

[ndeloof]

closes#3425

- What I did
introduce context show command, aligned with same command introduce in compose-cli

- How I did it
new cobra command, dump current context name after checking it exists

- How to verify it
docker context show

- Description for the changelog
introduce docker context show command

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Left some comments.

Now that we have the docker context show command, perhaps we should

• add a docs/reference/context_show.md file
• move the examples from https://github.com/docker/cli/pull/2500/files#diff-3589954942dcb3aa5aa11ca30bee4e904ebd9c35d1d9dfa73192f1655cdc5250R81
• (or keep the heading that's there, and link to the context show reference)

[codecov-commenter]

Codecov Report

Merging #3567 (c6b1518) into master (0b78efe) will decrease coverage by 0.01%.
The diff coverage is 27.77%.

@@ Coverage Diff @@## master #3567 +/- ##
==========================================
- Coverage 58.99% 58.98% -0.02% 
==========================================
Files 282 285 +3 Lines 23813 23834 +21 ==========================================
+ Hits 14049 14058 +9 - Misses 8908 8920 +12 
Partials 856 856 

[thaJeztah]

LGTM, thanks!