cmd: simplify create cluster logic

[gsora]

This PR refactors create cluster logic to make it slightly simpler.

New behaviors:

• if --split-existing-keys is enabled, --num-validators cannot be set
• --num-validators must be present unless --split-existing-keys is enabled, otherwise create cluster exits with error
• split key generation and disk read
• don't fill in with random keys when splitting: during --split-existing-keys before this PR, if the user specified a --num-validators greater than the amount of keys contained in --split-keys-dir Charon would fill in the blanks with random keys
• if definition file validator amount and --num-validators don't match, the creation process stops

category: refactor
ticket: #2341

[codecov]

Codecov Report

Patch coverage: 67.56% and project coverage change: +0.04 🎉

Comparison is base (305a2e5) 53.63% compared to head (ffe2253) 53.67%.

❗ Current head ffe2253 differs from pull request most recent head 2c689c2. Consider uploading reports for the commit 2c689c2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2380      +/-   ##
==========================================
+ Coverage   53.63%   53.67%   +0.04%     
==========================================
  Files         192      192              
  Lines       25931    25958      +27     
==========================================
+ Hits        13907    13933      +26     
- Misses      10299    10306       +7     
+ Partials     1725     1719       -6     

Impacted Files	Coverage Δ
cmd/createcluster.go	59.78% <67.56%> (+1.29%)	⬆️

... and 16 files with indirect coverage changes

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[OisinKyne]

Artificially set it to 1 if --split-existing-keys is enabled and user didn't provide --num-validators, so that the code flow can continue.

What happens if I enable --split-existing-keys and point --split-existing-keys-dir at a folder with a number of keys in it? Will it produce only 1, or will it know to produce as many validators as there are keystores?

[gsora]

I’m reworking the whole PR, I’ll ask for review later. The approach I used in this PR didn’t seem sound.

[gsora]

@OisinKyne please review the behavior changes now