Separate core `dotnetup` `record` classes

### Prerequisites

- [x] I have read the [dotnetup documentation](https://github.com/dotnet/sdk/tree/release/dnup/documentation/general/dotnetup).
- [x] I have searched for [existing dotnetup issues](https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3Adotnetup).
- [x] I have searched for [existing dotnetup discussions](https://github.com/dotnet/sdk/discussions/categories/dotnetup).

### Issue type

Bug report

### Description

`dotnetinstall.cs` contains `dotnetinstall` but also `dotnetinstallrequest` and other `record` types that are very useful and commonly referenced.
Each of those types should get their own file for simplicity and readability.

Often I develop in VS Code and VS Code can't properly load symbols to navigate to the types if I have not loaded the dotnetup workspace, so I try to ctrl t to the type but that also does not work unless the file name is loaded.
This will also reduce the risk of circular dependencies.

### dotnetup version

_No response_

Start in dotnetinstall.cs within the dotnetup workspace and identify dotnetinstallrequest and the other record types currently defined there. Give each record its own appropriately named file, then build the dotnetup workspace and confirm the types remain usable and navigable from VS Code.

Written by the indexing model from the issue text.