[EuiColorPicker] Need support for changing placeholder text and add isClearable option

[cchaos]

To help support a particular workflow in Lens where we have the idea of an "Auto" color, where then a user can override this, I've come up with a solution that doesn't involve a second click just for users to initiate "custom". This flow involves leaving the input blank to keep it in "Auto" mode, but the placeholder text says "Transparent" with no way to change it. Also, I'd like a quick single click action to return to "Auto" mode which would be by adding isClearable like we have in other inputs like search and combobox.

Here's an example in mocks

[cchaos]

I'm labelling this as urgent because this color picker is going in to Kibana 7.9, and without a really big hack/workaround to not use the color picker, we really need to be able to change the placeholder text (at the very least).