[feat]: SubNavHeader search's placeholder can't be customized

[ChristopheCVB]

A clear and concise description of what the feature is

Be able to set the SubNavHeader search field placeholder

design-system/packages/strapi-design-system/src/v2/SubNav/SubNavHeader.tsx

Line 88 in 3ad7300

placeholder="e.g: strapi-plugin-abcd"

Why should this feature be included?

The placeholder is currently statically set to e.g: strapi-plugin-abcd and is not really helpful

Please provide an example for how this would work

Add a searchPlacholder prop to the component and use it