Icons - add support for custom icons

Is this a new component or an extension of an existing one?
What is the existing component, if any?

Existing, MenuToggle, ToolbarFilter

When using PF tokens to create a custom theme, MenuToggle and ToolbarFilter use default icons in PF that currently cannot be changed via a prop since the prop does not exist, while components like Label, Alert, and Button, offer this functionality.

An alternative option was discussed at today's working session (cc @mcoker), which would include adding support for custom icons directly to react-icons and then importing any custom overrides.

Related: https://github.com/patternfly/patternfly-react/issues/10634

Describe the feature
A clear and concise description of the new feature. What is the expected behavior?

Are there visuals for this feature? If applicable, please include examples for each state and for varying widths

See below. MUI uses a different toggle icon for their menu toggle.

Include screenshots or links to Marvel or other mockups.
MUI:

PF:

Any other information?
Would it be possible to prioritize this work for an upcoming sprint to align with RHOAI priorities for the MUI theming effort @nicolethoen ?

Start by reviewing the MenuToggle and ToolbarFilter component implementations, then read related issue #10634 and the discussion about extending react-icons. Done means users can provide custom icons for both components through an agreed, documented API, with behavior consistent with the existing icon support in Label, Alert, and Button.