Proposal: Make preview features configurable

## Problem

A project I'm working on has an issue where a list of repos is returning a Server Error due to an additional `Accept` header being added (The "mediaTypeCodesOfConductPreview" header).

Removing this header fixes the problem but I can't do this with the current library implementation.

## Solution

Instead of adding these headers by default, we could move them to an Options parameter similar to ListOptions.

```go
type RequestOptions {
acceptHeaders []string
}
```

### Pros

- Allows developers to control which headers to add (including headers the library may not control)
- Removes the maintenance of adding / removing headers from library owners
- Opens the door for any additional request options in the future.
- This will remove the need for comments like `// TODO: remove custom Accept headers when APIs fully launch.`

### Cons

- Forces users to care about headers
- While a con, users should know if a feature they are using is preview or not and subject to change

## Concerns

The only major concern I have with this is approach is that it gets fuzzy what to do with entirely new API's (for example the checks API).

For consistency the API could still require the developer to add the header.
For a sane API the API could add the header with the expectation that the header is removed as soon as possible, but again moves the burden on to the library.

まず、デフォルトの preview Accept ヘッダーと ListOptions がどのように処理されるかを追跡します。issue にはファイルもテストも記載されていません。RequestOptions API を定義し、新しい API で preview ヘッダーをどのように扱うべきかを明確にしたうえで、repository-list リクエストから不要なヘッダーを省略できることを確認します。

索引モデルが issue の本文から書いたものです。