refactor(miniooni): divide et impera (#912)

This diff splits miniooni's implementation in smaller and more
easily tractable blocks ahead of future refactoring.

I'm trying to make `miniooni oonirun -i URL` as possible as
`miniooni -i URL oonirun`, because users typically expect this
kind of flexibity from modern Unix commands.

Part of https://github.com/ooni/probe/issues/2184

internal/cmd/miniooni/consent.go

@@ -0,0 +1,53 @@
+package main
+
+//
+// Acquiring user's consent
+//
+
+import (
+	"os"
+	"path"
+
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+)
+
+// acquireUserConsent ensures the user is okay with using miniooni. This function
+// panics if we do not have acquired the user consent.
+func acquireUserConsent(miniooniDir string, currentOptions Options) {
+	consentFile := path.Join(miniooniDir, "informed")
+	err := maybeWriteConsentFile(currentOptions.Yes, consentFile)
+	runtimex.PanicOnError(err, "cannot write informed consent file")
+	runtimex.PanicIfFalse(regularFileExists(consentFile), riskOfRunningOONI)
+}
+
+// maybeWriteConsentFile writes the consent file iff the yes argument is true
+func maybeWriteConsentFile(yes bool, filepath string) (err error) {
+	if yes {
+		err = os.WriteFile(filepath, []byte("\n"), 0644)
+	}
+	return
+}
+
+// riskOfRunningOONI is miniooni's informed consent text.
+const riskOfRunningOONI = `
+Do you consent to OONI Probe data collection?
+
+OONI Probe collects evidence of internet censorship and measures
+network performance:
+
+- OONI Probe will likely test objectionable sites and services;
+
+- Anyone monitoring your internet activity (such as a government
+or Internet provider) may be able to tell that you are using OONI Probe;
+
+- The network data you collect will be published automatically
+unless you use miniooni's -n command line flag.
+
+To learn more, see https://ooni.org/about/risks/.
+
+If you're onboard, re-run the same command and add the --yes flag, to
+indicate that you understand the risks. This will create an empty file
+named 'consent' in $HOME/.miniooni, meaning that we know you opted in
+and we will not ask you this question again.
+
+`