Support for customizing header component in Inbox

Added support for customizing the header component in inbox SDKs.

• @suprsend/react-inbox

You can now add a custom component to the right side of the header in the inbox popup. This replaces the "Mark all as read" text with any JSX you provide. You can even include custom icons, such as settings or preferences, in your JSX and use them to navigate users to specific pages. For an example, refer to https://docs.suprsend.com/docs/react-customize-inbox#adding-custom-header-component.

• @suprsend/web-inbox

In web-inbox, you can add an extra icon beside the "Mark all as read" button at the top of the inbox popup using headerIconUrl. You can also execute custom logic when this icon is clicked using headerIconClickHandler. This feature is useful for cases like displaying settings or preferences icons, which, when clicked, take users to the respective settings or preferences pages. For more information, refer to the documentation at https://docs.suprsend.com/docs/embeddable-inbox#customising-inbox.