Resolve CardDAV issues with Nextcloud vCards

[ViMaSter]

Currently, Linphone fails to connect to CardDAV servers that require credentials in plain text. Credentials are stored in an encrypted form and sent like that as well. This causes CardDAV servers like Nextcloud to fail to provide contact info.

Changes can best be reviewed per commit:

• 41e67af forces Linphone to use plaintext credentials stored, rather than encrypted values
• a49d960 resolves Nextcloud-specific vCard format issues
• ec298d1 applies Dial Plan Normalization; if a call is announced with international (+491234…, +11234…) or local format (01234…), Linphone resolves the contact, even if only one format is present in the vCard, by taking the international prefix from the Username and using it as a reference
• f3f2092 skips invalid properties of vCards, rather than failing to parse a contact's info altogether

Also requires changes to the desktop app at BelledonneCommunications/linphone-desktop#973

If both PRs are merged BelledonneCommunications/linphone-desktop#906 can be closed.

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Signed and sent the contributors' agreement yesterday.

[Viish]

Hi @ViMaSter,

Thanks for the PR.
We'll review it internally and let you know.

Best regards,