Add option to hide resources with no significant changes in plan output

[abrekhov]

Problem

When running nelm release plan install, resources with only insignificant changes are displayed with <hidden insignificant changes> placeholder:

┌ Update CustomResourceDefinition/gitlabs.apps.gitlab.com
│ <hidden insignificant changes>
└ Update CustomResourceDefinition/gitlabs.apps.gitlab.com
┌ Update Certificate/gitlab-serving-cert
│ <hidden insignificant changes>
└ Update Certificate/gitlab-serving-cert
...

In large releases with many resources, this creates a lot of noise, making it harder to spot actual significant changes.

Proposed Solution

Add a flag like --hide-unchanged or --only-significant that completely omits resources from the output when they have no significant diff (i.e., when uDiff is empty after filtering).

Currently in internal/plan/planned_changes.go:

if uDiff == "" {
    uDiff = HiddenInsignificantChanges
}

With the new flag, these resources could be skipped entirely from the output instead of showing the placeholder.

Expected Behavior

nelm release plan install --hide-unchanged chart.tgz

Would only show resources that have actual significant changes, improving readability.

[ilya-lesikov]

I'm not sure about hiding such resource changes completely, even with a flag. I think it's important to still see the list of resources with insignificant changes somewhere in the output. Instead I propose something like this:

┌ Update Certificate/other-cert
│ - removed
│ + added
│ + added
└ Update Certificate/other-cert

Other changes:
- Create CustomResourceDefinition/gitlabs.apps.gitlab.com (hidden insignificant changes)
- Update Certificate/gitlab-serving-cert (hidden sensitive changes)

... instead of the current:

┌ Update Certificate/other-cert
│ - removed
│ + added
│ + added
└ Update Certificate/other-cert

┌ Update CustomResourceDefinition/gitlabs.apps.gitlab.com
│ <hidden insignificant changes>
└ Update CustomResourceDefinition/gitlabs.apps.gitlab.com

┌ Update Certificate/gitlab-serving-cert
│ <hidden sensitive changes>
└ Update Certificate/gitlab-serving-cert

This way you still see what will happen with the resource, but without the unnecessary verbosity.

[abrekhov]

In big charts the diff in charts become too big, even when you just change one parameter.

All the insignificant changes usually are some artifacts of server side apply which is not a change at all from engineer's perspective.

It might be optional flag disabled by default. I'm sure that a lot of people wonder to see the nelm as terraform for charts with the short and simple diff.

[ilya-lesikov]

@abrekhov I think my proposed solution is good enough. It decreases verbosity by 4 times (counting the number of lines), requires no configuration, and improves the default behavior too.

All the insignificant changes usually are some artifacts of server side apply which is not a change at all from engineer's perspective.

Most of the time, but not always. --runtime-annotations, --runtime-labels, werf.io and helm.sh annotations, runtime werf annotations: they might be hidden there too. Also, other functionality might be added later which makes some other field changes display as insignificant