add user exit

Author: christianguenter2

docs/advanced/extensibility/user_exits.md

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+You can extend abap2UI5 with predefined user exits. The user exits are exposed by the interface `Z2UI5_IF_EXIT`. To use the user exits in your system you have to create a new class which implements the interface and its methods. They're called dynamically by abap2UI5 class `Z2UI5_CL_EXIT`. You should **not** include your class into abap2UI5 packages but in any other custom package.
+
+These are the available methods:
+- `get_draft_exp_time_in_hours`: You can override the default 4 hour abap2UI5 draft session timeout
+- `adjust_config`: You can change abap2UI5 startup parameters like the theme or UI5 version