Add "delimited" prop to EuiComboBox and support copy/paste

[cjcenizal]

Per #1307 (comment):

I think the combo box could accept a prop with the character(s) that would trigger an entry creation. It is very common in those inputs to trigger with the , (comma).

The interface would look like this:

<EuiComboBox delimiter="," />

The supported behaviors would be:

1. Entering a comma will behave as if the user has hit Enter, and a new option based on the user input will be created.
2. Pasting any values into the combo box will be split on the delimiter, and valid values will be converted into options. Invalid values will be left as a string in the input.
3. The user can click a "Copy" icon, which will copy a delimited string of the pill values to the clipboard. The user can highlight all and copy as a delimited string

[anishagg17]

@cchaos please assign me this issue