[web] introduce secondary button prop to modal component

Summary:
Sometimes we need two buttons in our modals. This diff introduces a secondary button prop which will enfore style rules when there are two buttons in the modal.
The style rules for when there is a primary + secondary button are as follows:

• If there are two buttons needed for a single modal, the secondary button may live in the button section next to the main button.
• The secondary button can be outlined to show less visual hierarchy.
• The buttons will only be as wide as the button’s text content and lie on the right side of the modal.

Here is a screenshot from figma showing how modals with primary + secondary should look visually:

Linear task: https://linear.app/comm/issue/ENG-5943/extendmodify-the-modal-props-api-to-follow-new-modal-designs

Depends on D10242

Test Plan:
Here is an example of using the secondaryButton prop

Reviewers: atul, rohan, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10266