Payload encoding detection

[guilhem]

Feature Request

Environment

I'm using flatcar on Akamai Linode

Desired Feature

Payload is limited in size.
Currently, we are uploading ignition.json with a base64 encoding.
But it can be oversize the limit, so we are/want to use a gzip content.

I see VMware have already this:

ignition/internal/providers/vmware/vmware.go

Lines 33 to 55 in 3b3a27b

	func decodeConfig(config config) ([]byte, error) {
	switch config.encoding {
	case "":
	return []byte(config.data), nil
	case "b64", "base64":
	return decodeBase64Data(config.data)
	case "gz", "gzip":
	return decodeGzipData(config.data)
	case "gz+base64", "gzip+base64", "gz+b64", "gzip+b64":
	gz, err := decodeBase64Data(config.data)
	if err != nil {
	return nil, err
	}
	return decodeGzipData(string(gz))
	}
	return nil, fmt.Errorf("unsupported encoding %q", config.encoding)
	}

And other ticket are for specific providers #1096

Can we add a generic way to detect payload (b64 / gzip+b64 / zstd+b64...)

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