CR 0007735: Introduction of additional SIP message header fields and MIME_Encapsulated_Part

Test cases for IMS eCall and MCPTT have been requested by RAN5, which require updates to LibSip and MIME body type definitions by 3GPP TF160

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New eCall TCs require an update of MIME type defs :
- Container for MSD data
- MIME field Content-ID
See R5-174537 based on main eCall RFC 8847

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MCPTT default message contents in R5-173766 (TS 36579-1 v.0.2.0) require following new headers:
- Answer-Mode RFC 5373
- Priv-Answer-Mode RFC 5373
- Resource-Priority RFC 4412
- Target-Dialog RFC 4538
- P-Answer-State RFC 4964