Adding emergency registration procedure

[kbaccar]

• Added IMEI based emergency registration for srsUE
• Added a new parameter nas.enable_emergency_reg (default = false)

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:white_check_mark: kbaccar
:x: Karim Baccar

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[andrepuschmann]

Hey @kbaccar, thanks for the PR again. We'll release the 23.04 version of srsRAN 4G in a few days that will include a few changes in the files your PR touched. Would be good if you could rebase your PR on top of it once it's out.

[kbaccar]

@andrepuschmann Conflict fixed, branch have been merged to master.