For #21084: add Runtime.execReadStandardOutOrThrow.

This command makes it a one liner to execute an external command, which will
allow us to easily run git commands in the next section.

Sample error output:

```
* What went wrong:
Execution failed for task ':app:gitHash'.
> command exited with non-zero exit value: 1.
  cmd: git reev-parse --short HEAD
  stderr:
  git: 'reev-parse' is not a git command. See 'git --help'.

  The most similar command is
        rev-parse
```

buildSrc/src/main/java/org/mozilla/fenix/gradle/ext/Runtime.kt

@@ -0,0 +1,29 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.gradle.ext
+
+import java.util.concurrent.TimeUnit
+
+/**
+ * Executes the given command with [Runtime.exec], throwing if the command returns a non-zero exit
+ * code or times out. If successful, returns the command's stdout.
+ *
+ * @return stdout of the command
+ * @throws [IllegalStateException] if the command returns a non-zero exit code or times out.
+ */
+fun Runtime.execReadStandardOutOrThrow(cmd: Array<String>, timeoutSeconds: Long = 30): String {
+    val process = Runtime.getRuntime().exec(cmd)
+
+    check(process.waitFor(timeoutSeconds, TimeUnit.SECONDS)) { "command unexpectedly timed out: `$cmd`" }
+    check(process.exitValue() == 0) {
+        val stderr = process.errorStream.bufferedReader().readText().trim()
+        """command exited with non-zero exit value: ${process.exitValue()}.
+           |cmd: ${cmd.joinToString(separator = " ")}
+           |stderr:
+           |${stderr}""".trimMargin()
+    }
+
+    return process.inputStream.bufferedReader().readText().trim()
+}