Add props property to add_avb_hash_footer

It is used to provide name:value properties to the footer. Value can be
from a text in *.bp file or a binary file referenced via the `file`
prop. e.g.

```
avb_add_hash_footer {
	...
	props: [
		{
			name: "string_prop",
			value: "string_value",
		},
		{
			name: "binary_prop",
			file: "a_binary_file_name",
		},
	],
}
```

This CL also adds a test for the module type which has been missing.

Bug: 256148237
Test: m nothing
Change-Id: Idf55b308c8ce760387c01a847846b42d1aebe4ea

filesystem/filesystem.go

@@ -34,6 +34,7 @@ func init() {
 func registerBuildComponents(ctx android.RegistrationContext) {
 	ctx.RegisterModuleType("android_filesystem", filesystemFactory)
 	ctx.RegisterModuleType("android_system_image", systemImageFactory)
+	ctx.RegisterModuleType("avb_add_hash_footer", avbAddHashFooterFactory)
 }
 
 type filesystem struct {