EuiModal add height or min-height prop

[ryankeairns]

Canvas has a few instances where modals display data that can be filtered via a search input which, in turn, reduces the height of the euiModalBody content and thus the modal itself. This results in a rather jumpy experience as seen here --> https://github.com/elastic/kibana-canvas/issues/869

[ryankeairns]

In the end #1165, we opted to only add a maxWidth prop leaving other style overrides (e.g. width, height, max-height) to be done via style or CSS classes.