feat: completion for the Fish shell

[morozov]

Description
This patch enables Skaffold to generate completion for the Fish shell.

User-facing changes (remove if N/A)
Currently, there's no out-of-the-box completion for the Fish shell, so it needs to be generated manually.

With these changes, the completion looks like the following:

→ ~ skaffold <TAB>
apply                                                  (Apply hydrated manifests to a cluster)
build                                                                    (Build the artifacts)
completion                   (Output shell completion for the given shell (bash, fish or zsh))
config  (Interact with the global Skaffold config file (defaults to `$HOME/.skaffold/config`))
debug                                                           (Run a pipeline in debug mode)
delete                                             (Delete any resources deployed by Skaffold)
deploy                                                            (Deploy pre-built artifacts)
dev                                                       (Run a pipeline in development mode)
diagnose                                                        (Run a diagnostic on Skaffold)
fix                                       (Update old configuration to a newer schema version)
help                                                                  (Help about any command)
init                                     (Generate configuration for deploying an application)
options
render                                                (Generate rendered Kubernetes manifests)
run                                                                           (Run a pipeline)
schema                        (List JSON schemas used to validate skaffold.yaml configuration)
survey                                   (Opens a web browser to fill out the Skaffold survey)
test                                         (Run tests against your built application images)
verify                                   (Run verification tests against skaffold deployments)
version                                                        (Print the version information)

[conventional-commit-lint-gcf]

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[codecov]

Codecov Report

Merging #8097 (10cb04f) into main (290280e) will decrease coverage by 4.01%.
The diff coverage is 53.26%.

@@            Coverage Diff             @@
##             main    #8097      +/-   ##
==========================================
- Coverage   70.48%   66.47%   -4.02%     
==========================================
  Files         515      597      +82     
  Lines       23150    29174    +6024     
==========================================
+ Hits        16317    19393    +3076     
- Misses       5776     8339    +2563     
- Partials     1057     1442     +385     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/completion.go	13.04% <0.00%> (-1.25%)	⬇️
cmd/skaffold/app/cmd/credits/export.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/deploy.go	40.90% <0.00%> (-12.94%)	⬇️
cmd/skaffold/app/cmd/test.go	44.44% <0.00%> (ø)
cmd/skaffold/app/exitcode.go	100.00% <ø> (+6.66%)	⬆️
cmd/skaffold/skaffold.go	0.00% <0.00%> (ø)
cmd/skaffold/app/cmd/inspect_tests.go	62.50% <14.28%> (-1.14%)	⬇️
cmd/skaffold/app/cmd/render.go	35.48% <18.18%> (-5.90%)	⬇️
cmd/skaffold/app/cmd/lsp.go	28.12% <28.12%> (ø)
cmd/skaffold/app/cmd/run.go	64.28% <33.33%> (-9.63%)	⬇️
... and 394 more

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[gsquared94]

lgtm, thanks!