res_pjsip: Replace invalid UTF-8 sequences in callerid name

* Added a new function ast_utf8_replace_invalid_chars() to
  utf8.c that copies a string replacing any invalid UTF-8
  sequences with the Unicode specified U+FFFD replacement
  character.  For example:  "abc\xffdef" becomes "abc\uFFFDdef".
  Any UTF-8 compliant implementation will show that character
  as a � character.

* Updated res_pjsip:set_id_from_hdr() to use
  ast_utf8_replace_invalid_chars and print a warning if any
  invalid sequences were found during the copy.

* Updated stasis_channels:ast_channel_publish_varset to use
  ast_utf8_replace_invalid_chars and print a warning if any
  invalid sequences were found during the copy.

ASTERISK-27830

Change-Id: I4ffbdb19c80bf0efc675d40078a3ca4f85c567d8