Enhancement: Add User-Defined Keyword List for Notifications

### Your use case

#### What would you like to do?

I would like to have the ability to create a user-defined list of notification keywords.

#### Why would you like to do it?

Currently, whilst Element X (for Android) allows you to set up notifications for keywords, it doesn't provide a means to define such keywords.

#### How would you like to achieve it?

### Have you considered any alternatives?

There are no workarounds or alternatives to such functionality at present in Element X (for Android).

### Additional context

I have created a [parent discussion](https://github.com/element-hq/element-meta/discussions/2988) regarding the lack of such functionality in Element X.

### Are you willing to provide a PR?

No

Start by reviewing the linked parent discussion to understand the proposed user-defined keyword behavior and scope for Element X on Android. Done means users can define notification keywords and those keywords are used for notifications, but this issue names no files, tests, or implementation entry points.

Written by the indexing model from the issue text.