[SDK-338] Inbox Header Customization

[franco-zalamena-iterable]

🔹 Jira Ticket(s) if any

• SDK-338

✏️ Description

This Pr is meant to add easy out of the box customization for the Inbox Header.
Currently we only have a modifiable title in the inbox, which is a bit limited, if someone just wants a back button that requires a whole wrapper on the fragment or recreating the whole fragment just for that.
To address this concern it is now possible to opt-in two pre built headers and customizable one.

• Default: Just title
• WithBackButton: Title and a left back button
• Custom: You can pass a layout resource to be rendered in the header, this layout can contain Iterable specific ids to add actions and automatic customization from the fragment

Default	WithBackButton

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Things to consider:

• Backward compatible: None is the default for every existing entry point, current integrators see no change.
• Tests: 13 unit tests in iterableapi-ui (Robolectric, new module test harness). Covers all four variants and apply() state transitions.