Allow customising chevron colour

[Michael Voong]

Is your feature request related to a problem? Please describe.
Currently, there is no way of customizing the chevron colour, so it's difficult to get a dark mode looking right.

Describe the solution you'd like
A separate property added to CheckoutTheme for altering the chevron tint color.

Describe alternatives you've considered
Trying to access the underlying UIButton and changing the tint colour of the image view.

Additional context
The disclosure indicator chevron next to "Billing Details*" is black and cannot be customised:

[deepesh-vasthimal-cko]

@mvoong this is now released : https://github.com/checkout/frames-ios/releases/tag/3.3.0. Any questions please feel free to reopen the issue. Once again thank you for creating the PR.

[Michael Voong]

Thanks, we've integrated with this already and looks good.