[i269] Add theming for reaction and message options in ChatTheme

[liviu-timar]

🎯 Goal

Add new theme properties to ChatTheme that control visibility of reaction and message option lists in the selected message menu.

Closes:

• https://github.com/GetStream/android-internal-board/issues/269

🛠 Implementation details

• Added two new ChatTheme properties: reactionOptionsTheme and messageOptionsTheme.
• Used these new properties in SelectedMessageMenu.headerContent and in defaultMessageOptionsState() to control option visibility.
• Documented new features.

🧪 Testing

Pass custom values to the new ChatTheme parameters in the sample app and check if reaction & message options are visible/hidden when long tapping on a message.

☑️Contributor Checklist

General

• I have signed the Stream CLA (required)
• Assigned a person / code owner group (required)
• Thread with the PR link started in a respective Slack channel (#android-chat-core or #android-chat-ui) (required)
• PR targets the develop branch
• PR is linked to the GitHub issue it resolves

Code & documentation

• Changelog is updated with client-facing changes
• Affected documentation updated (KDocs, docusaurus, tutorial)

☑️Reviewer Checklist

• UI Components sample runs & works
• Compose sample runs & works
• UI Changes correct (before & after images)
• Bugs validated (bugfixes)
• New feature tested and works
• Release notes and docs clearly describe changes
• All code we touched has new or updated KDocs