ref(cmd/cli): isolate install validation logic

[michelleN]

I was reviewing this code with the new folks on the team and realized it could use a little re-organization to make it easier to understand what is happening.

• No functional code changes
• Adds comments in line

Please mark with X for applicable areas.

• New Functionality [ ]
• Documentation [ ]
• Install [ ]
• Control Plane [ ]
• CLI Tool [X]
• Certificate Management [ ]
• Networking [ ]
• Metrics [ ]
• SMI Policy [ ]
• Security [ ]
• Tests / CI System [ ]
• Other [ ]

Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution? No

[codecov-commenter]

Codecov Report

Merging #1621 into main will increase coverage by 0.00%.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##             main    #1621   +/-   ##
=======================================
  Coverage   55.05%   55.06%           
=======================================
  Files         117      117           
  Lines        4922     4949   +27     
=======================================
+ Hits         2710     2725   +15     
- Misses       2210     2222   +12     
  Partials        2        2           

Impacted Files	Coverage Δ
cmd/cli/install.go	66.20% <87.50%> (+2.02%)	⬆️
...ertificate/providers/tresor/certificate_manager.go	65.34% <0.00%> (-11.89%)	⬇️
pkg/envoy/rds/response.go	26.86% <0.00%> (-0.41%)	⬇️
pkg/catalog/routes.go	81.60% <0.00%> (+2.41%)	⬆️

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