func_odbc: Add SQL_ESC_BACKSLASHES dialplan function.

Some databases depending on their configuration using backslashes
for escaping. When combined with the use of ' this can result in
a broken func_odbc query.

This change adds a SQL_ESC_BACKSLASHES dialplan function which can
be used to escape the backslashes.

This is done as a dialplan function instead of being always done
as some databases do not require this, and always doing it would
result in incorrect data being put into the database.

ASTERISK-29838

Change-Id: I152bf34899b96ddb09cca3e767254d8d78f0c83d