cc-select: introduce a readonly property

[florian-sanders-cc]

Context

With #1102, we decided to stop relying on the disabled attribute during API calls and use the readonly (for cc-input-* components) / waiting (for cc-button) attribute instead.

We need to have the same logic for the cc-select component but it doesn't expose any readonly attribute at the moment.

We need to add a readonly attribute / property to the cc-select component.

[pdesoyres-cc]

See below what we need to do to make a select readonly:

clever-components/src/components/cc-kv-explorer/cc-kv-explorer.js

Line 527 in 7c49b53

// todo: remove this filter and replace by a proper `readonly` property once `<cc-select>` supports it.