Provide extension point to add a custom menu to the left sidebar

[Fox32]

Is your feature request related to a problem? Please describe.

We currently fork Element to add a custom menu to the left sidebar. It is used to jump to other web application in our setup.

We would prefer if we can perform this change without forking Element.

Describe the solution you'd like

An extension point that allows to places buttons in the left sidebar (SpacePanel) and show a menu on click. In addition:

• We want to provide a custom icon to the button in the sidebar
• We want to provide grouped menu entries or headings to structure the menu
• We want to provide custom icons for each menu entry
• We want to provide a href for each menu entry to link to another web page (maybe also an on click handler, but not for our use case).

Describe alternatives you've considered

We assume that our use case is to special to be integrated into Element Web directly, e.g. via a list of links in the configuration. In our case the list of menu items is loaded dynamically for the user via a backend.

Additional context
I can provide additional details, e.g. the exact changes in out fork privately if you are interested.