Resolves https://github.com/tuist/tuist/issues/354
### Short description 📝
Add support to specify array of paths / glob patterns for headers such as the case below:
```
Headers(public: ["Sources/A/**/*.h", "Sources/B/**/*.h"],
private: ["Sources/C/**/*.h", "Sources/D/**/*.h"],
project: "Sources/E/**/*.h")
```
### Solution 📦
Similar to the way multiple source file paths/glob patterns were implemented (https://github.com/tuist/tuist/pull/266). We can use `FileList` and `ExpressibleByStringLiteral` to expose public, private and project headers as either a String or Array of Strings. The original `FileList` used for sources was not reused as it contained `compilerFlags` which is specific to source files. A new `FileList` containing only paths/glob patterns was created for use with headers. This way `FileList` can be potentially reused elsewhere.
### Implementation 👩💻👨💻
- [X] Remove `FileList` type alias from `SourceFilesList` and references to it
- [X] Create new `FileList` without `compilerFlags`
- [X] Modify `Headers` to use the new `FileList` for public, private and project headers
- [X] Add tests to `GeneratorModelLoaderTests` to verify newly supported cases work
- [X] Create `ios_app_with_headers` fixture to support new array of strings case for headers
- [X] Write test that verifies `ios_app_with_headers` fixture targets build
### Test Plan
- Run `tuist generate` inside the `ios_app_with_headers` fixture
- Verify that the Framework1 target has the correct header files specified in the manifest
Pedro Piñera