[iOS] Fix to be able to use custom font for contributors screen of iOS

[tkhs0604]

Idea Description
Currently, we are applying a workaround to use a system default for iOS side.
We would like to load the custom font (Monteserrat) file from the resource directory and use it.

Reference images and links

• Actual function for iOS
  • https://github.com/DroidKaigi/conference-app-2023/blob/169f2d129a35033b2e4f8399d7c156252769b0f4/core/designsystem/src/iosMain/kotlin/io/github/droidkaigi/confsched2023/designsystem/theme/FontResource.kt
• Custom font file
  • https://github.com/DroidKaigi/conference-app-2023/blob/f0c42e8ab47e3dbe553c0f2e3b2fd95f69f6681c/core/designsystem/src/commonMain/resources/font/montserrat_medium.ttf