tags: fix index() lifetime bind

The signature for `TagListRef::index` didn't bind the lifetime of the returned
`TagValue` to `&self`. This causes the following code to compile:

```rust
 1 let title = {
 2     let mut tags = TagList::new();
 3     {
 4         let tags = tags.get_mut().unwrap();
 5         tags.add::<Title>(&"some title", TagMergeMode::Append);
 6     }
 7
 8     let title = tags.index::<Title>(0).unwrap();
 9     assert_eq!(title.get(), "some title");
10
11     title
12 };
13
14 assert_eq!(title.get(), "some title");
```

... but it panics at runtime on the last `title.get()`:

```
Invalid tag type: WrongValueType(ValueTypeMismatchError
    { actual: <invalid>, requested: gchararray })
```

Indeed, the `title` `TagValue` is freed with the `tags` on line 12.

This commit fixes the function signature so the returned `TagValue` can't
outlive its `TagListRef`.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1520>

gstreamer/src/tags.rs

@@ -483,7 +483,7 @@ impl TagListRef {
 
     #[doc(alias = "get_index")]
     #[doc(alias = "gst_tag_list_get_index")]
-    pub fn index<'a, T: Tag<'a>>(&self, idx: usize) -> Option<&'a TagValue<T::TagType>> {
+    pub fn index<'a, T: Tag<'a>>(&'a self, idx: usize) -> Option<&'a TagValue<T::TagType>> {
         self.index_generic(T::TAG_NAME, idx).map(|value| {
             if !value.is::<T::TagType>() {
                 panic!(