[API Proposal]: `Concat` support for `IncrementalValuesProvider<T>`

### Background and Motivation

When the values are collected from different sources, it's useful to be able to concat all of them so we can generate a single file for both.

### Proposed API

```cs
public static IncrementalValuesProvider Concat(this IncrementalValuesProvider, IncrementalValuesProvider)
```

### Usage Examples
```cs
typeSymbolsFromMetadata.Concat(typeSymbolsFromSource)
```