Use spacing BD values for FlexGap, GridGap, etc

[MariaJose]

Is your feature request related to a problem? Please describe.
Right now we are using css values for FlexGap, GridGap, etc. However, would be good to use BD spacing values when using this props for Flex and Grid components.

Describe the solution, feature, or improvement you'd like
Use Spacing BD values for FlexGap, FlexColumnGap, FlexRowGap, GridGap, GridColumnGap, and GridRowGap props.

References:
Spacing values in BD: https://developer.bigcommerce.com/big-design/spacing
Flex component: https://developer.bigcommerce.com/big-design/flex
Grid component: https://developer.bigcommerce.com/big-design/grid

Example:

How it can be used for now:

<Flex flexGap="1rem" />

Suggestion:

<Flex flexGap="small" />