PF4: Custom Select Component: Single Select variation

[tlabaj]

This issue covers the Single Select variation of the Custom Select only:
(This issue is a child of issue #1206)

Related issue in Core: patternfly/patternfly#651

Info provided on the component: https://docs.google.com/document/d/1rrCdARutABI-Vpa_mdMWGl2xJJTtt2dPQ2bjFx2zwCM/edit?usp=sharing

see: https://pf-next.com/components/Select/examples/

In addition to a React review, this issue should be reviewed by the core contributors:
CSS: @srambach (CSS) @mcarrano (interaction design) @christiemolloy (visual design)

[rachael-phillips]

Closing this issue because it was completed in PR #1277