feat: initial choice of exec command

[robertpsoane]

• First pass at offering user choice as to exec command
• There is an open question as to whether or not to offer a "default exec" action along with a "detailed exec" action
• This also forms the basis of how I might implement the run container feature - though this will need some sort of scrolling
• Also want to add some sort of "single-use-autocompletion" wherein the default exec command is faded but can be tabbed to completion

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 106 lines in your changes missing coverage. Please review.

Files	Coverage Δ
src/components/text_input_wrapper.rs	0.00% <0.00%> (ø)
src/pages/describe.rs	0.00% <0.00%> (ø)
src/widgets/text_input.rs	0.00% <0.00%> (ø)
src/docker/container.rs	0.00% <0.00%> (ø)
src/pages/attach.rs	0.00% <0.00%> (ø)