Add helm-v4 support

[avoidik]

• Deprecate helm-v2
• Add aarch64/arm64 support to Makefile
• Correct plugin manifest

ref. #225

$ make build
$ make install
$ helm plugin list
$ helm cm-push --help

[nerdeveloper]

Thank you for this PR I will take a look

[avoidik]

Thanks! I will have to rebase first

[FLimburg]

Thanks @avoidik, added it to our fork.

[avoidik]

hello @nerdeveloper I have rebased my PR, now it's ready for review

[avoidik]

By default plugin.yaml uses helm v4 spec

[nerdeveloper]

Overall the Helm 4 support implementation looks solid with proper backward compatibility for Helm 3. Found a few minor issues:

1. Bug: Error handling in certPoolFromFile function

Files: cmd/helm-cm-push/main_test.go (line 37) and pkg/chartmuseum/upload_test.go (line 33)

if !caCertPool.AppendCertsFromPEM(caCert) {
    return nil, err  // BUG: err is nil here from previous successful ReadFile
}

Fix: Return a descriptive error instead:

if !caCertPool.AppendCertsFromPEM(caCert) {
    return nil, fmt.Errorf("failed to append CA certs from PEM file: %s", filename)
}

2. Typo in test file

File: pkg/helm/chart_test.go (lines 36, 39)

expexted should be expected