Gem::RemoteFetcher uses Gem::Request, which adds the RubyGems UA.
Gem::RemoteFetcher is used to download gems, as well as the full index.
We would like the bundler UA to be used whenever bundler is making
requests.

This PR also avoids unsafely mutating the headers hash on the shared
`Gem::RemoteFetcher.fetcher` instance, which could cause corruption or
incorrect headers when making parallel requests. Instead, we create one
remote fetcher per rubygems remote, which is similar to the connection
segregation bundler is already doing

https://github.com/rubygems/rubygems/commit/f0e8dacdec

Before:

```
Fetching gem metadata from https://rubygems.org/........
Fetching source index from https://rubygems.pkg.github.com/my-org/

Bad username or password for https://x-access-token@rubygems.pkg.github.com/my-org/.
Please double-check your credentials and correct them.
```

After:

```
Fetching gem metadata from https://rubygems.org/........
Fetching source index from https://rubygems.pkg.github.com/my-org/

Access token could not be authenticated for https://x-access-token@rubygems.pkg.github.com/my-org/.
Make sure it's valid and has the necessary scopes configured.
```

https://github.com/rubygems/rubygems/commit/2ae69c964a

  Picked at 12aeef6ba9a3be0022be9934c1a3e4c46a03ed3a

https://github.com/bundler/bundler/commit/7eccba0e52

Windows paths do not start with a slash, so we add an extra slash to
separate the host from the path in file:// urls. Otherwise "D:" is
parsed as the host segment in the URI.

The path for those URLs now starts with "/", so we ignore that leading
character when using the URI's path.

This reduces Windows CI spec failures from 429 to 355.

https://github.com/bundler/bundler/commit/1b7e274cbc

  Pick from 8dd59e3ba97eb80a599f8149f31bf40773b69dc0