feat: Add --threshold flag to nomos vet

[karlkfi]

• Added a new --threshold[=MAX] option to the nomos vet command that allows specifying a maximum number of objects. If set, it will enable validation that will error if the number of objects exceeds the specified value, after rendering and cluster selectors. By default, this validation is disabled. If you pass the option without a value, the default value of 1000 is used.
• The value 1000 was chosen as a compromise between scale and safety. Technically we know form e2e tests that the inventory can actually hold at least 5000 objects without needing to disable the status, however, this is unsafe to do in production because each of those objects could error, which adds error conditions to the inventory status, which can significantly increase the size of the inventory.
• Unlike other options, this option has an optional value and different defaults when specified than when not specified. This requires the flag and value to be sent in the same argument, like --threshold=1000, instead of as two different arguments, like --threshold 1000.
• If the option is specified with a value of zero or lower, the validation will be disabled, the same as if the option was not specified.

This replaces the PoC PR: #1531

TODO: write some tests

[google-oss-prow]

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