allow calendar navigation button variant styling

[darrinmn9]

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

It seems that b-calendar does not support the styling of the date navigation buttons, and always applies "outline-secondary" to the button styling. Normally this isn't an issue, but when using b-calendar with date navigation button slots, I'd like to have the ability to change the button variant instead of it being forced to "outline-secondary".

In the screenshot below:

my custom theme uses primary - blue, secondary - orange
I added custom svgs for the date navigation button slots that match the primary variant. This creates an undesired orange background color on hover. I can't find any way to change this via current props of b-calendar.

Describe the solution you'd like

Add a prop to control the date navigation button variant nav-button-variant (default to "outline-secondary" so that this feature can be added in a non-breaking way)

[darrinmn9]

ty for such a quick PR @Hiws