cloud: DetectConfigChangeType helper

This aims to encapsulate the somewhat-weird logic we currently use to
distinguish between the various "terraform init" situations involving
Terraform Cloud mode, in the hope of making codepaths that branch based
on this slightly easier to read.

This isn't yet used, but uses of it will follow in subsequent commits.

internal/cloud/configchangemode_string.go

@@ -0,0 +1,37 @@
+// Code generated by "stringer -type ConfigChangeMode"; DO NOT EDIT.
+
+package cloud
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[ConfigMigrationIn-8600]
+	_ = x[ConfigMigrationOut-8598]
+	_ = x[ConfigChangeInPlace-8635]
+	_ = x[ConfigChangeIrrelevant-129335]
+}
+
+const (
+	_ConfigChangeMode_name_0 = "ConfigMigrationOut"
+	_ConfigChangeMode_name_1 = "ConfigMigrationIn"
+	_ConfigChangeMode_name_2 = "ConfigChangeInPlace"
+	_ConfigChangeMode_name_3 = "ConfigChangeIrrelevant"
+)
+
+func (i ConfigChangeMode) String() string {
+	switch {
+	case i == 8598:
+		return _ConfigChangeMode_name_0
+	case i == 8600:
+		return _ConfigChangeMode_name_1
+	case i == 8635:
+		return _ConfigChangeMode_name_2
+	case i == 129335:
+		return _ConfigChangeMode_name_3
+	default:
+		return "ConfigChangeMode(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+}