Create Roslyn analyzer for converting runtime `Enum.ToString()` calls to `nameof(Enum)`

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Task description

When looking through the codebase, I noticed a few places where we are doing:

```c#
field.PutAttribute(legacyKey, LegacyDocValuesType.BYTES_FIXED_DEREF.ToString());
```

The `BYTES_FIXED_DEREF` is a compile-time enum value, so it would both be more readable, and save an initial allocation if we changed it to:

```c#
field.PutAttribute(legacyKey, nameof(LegacyDocValuesType.BYTES_FIXED_DEREF));
```

I had ChatGPT sketch up the code for what this analyzer and code fix would look like:

https://chatgpt.com/share/69f41082-6ccc-8323-8639-9d4b6ff906f3

Note that this won't work on runtime values of variables that are declared `LegacyDocValuesType`, it is only sensible to do this when referencing the enum value directly as in the above example.

Start by reviewing the linked Roslyn analyzer and code-fix sketch, then locate the project's analyzer entry points and test coverage. Validate the direct enum-member example, while ensuring runtime enum variables are excluded; done means the analyzer offers the conversion and its tests cover the expected diagnostic and fix.

Written by the indexing model from the issue text.