E1AP enc/dec lib improvements

This MR is including some fixes and improvements for the E1AP library,
focusing mostly on Bearer Context Management:

- struct reorganization: split setup and modification request messages
  into distinct structures, ensuring compliance with 3GPP TS 38.463
  specifications
- add encoding/decoding sub-functions for different IEs
- improvements to Bearer Context Management: Implemented various
  improvements to the E1 Bearer Context Setup Response and Request,
  including the adoption of encoding/decoding sub-functions for
  different IEs
- use lib functions whenever need (e.g. cp_bearer_context_setup_request
  in cucp_cuup_e1ap.c)
- handle optional struct members as pointers
- updated copy/equality check utility functions

EDIT: The MR is also including the changes from !3222 (closed):

> While testing E1 with security enabled for DRBs, it was found that
> data traffic was not functional.
>
> The problem was that security settings were modified in the PDCP
> reestablishment happening when receiving Bearer Context Modification
> Request.
>
> So some restructuring was done.
>
> The actual fix is in 18176297, the others are for support. Basically
> we introduce a boolean to check if the security settings are present in
> Bearer Context Modification Request and we change PDCP security settings
> only if they are.
>
> (Note that in OAI CU security settings are not sent in Bearer Context
> Modification Request, so we could also remove all this, but we need to
> be compatible with other cu-up and/or cu-cp.)