PF4: Custom Select Component

[rachael-phillips]

This issue will be complete once all the variations of the Custom Select have been implemented.

• Single Select, issue PF4: Custom Select Component: Single Select variation #1214
• Checkbox select (multiple items), issue PF4: Custom Select Component: Checkbox select (multiple items) variation #1217
• Type-ahead select (single item), issue PF4: Custom Select Component: Type-ahead select (single item) #1215
• Type-ahead select (multiple items), issue PF4: Custom Select Component: Type-ahead select (multi item) variation #1216

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)