feat: add shell mode

[Codelax]

TODO:

• A correct shell command
• Ctrl-C doesn't work after quitting shell
• use [namespace] command
• in suggestions, put required options first
• Allow switching profile and display used profile
• Fix crash with invalid command
• Refactor

[codecov-commenter]

Codecov Report

Merging #2433 (c0a6186) into master (9681ee8) will decrease coverage by 0.58%.
The diff coverage is 11.88%.

@@            Coverage Diff             @@
##           master    #2433      +/-   ##
==========================================
- Coverage   76.45%   75.87%   -0.59%     
==========================================
  Files         125      126       +1     
  Lines       21570    21762     +192     
==========================================
+ Hits        16492    16511      +19     
- Misses       4521     4693     +172     
- Partials      557      558       +1     

Impacted Files	Coverage Δ
internal/core/command.go	57.39% <0.00%> (-2.61%)	⬇️
internal/core/shell.go	10.49% <10.49%> (ø)
internal/core/bootstrap.go	72.46% <20.00%> (-2.36%)	⬇️
...nal/namespaces/instance/v1/custom_server_create.go	81.94% <36.36%> (-0.35%)	⬇️

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