SEC-1915 Add cutomisation of search filter in ActiveDirectoryLdapAuthenticationProvider

The default AD search filter is currently hardcoded but in some setups
needs to be adjusted.
Adding two properties to ActiveDirectoryLdapAuthenticationProvider:
- searchFilter - the LDAP search filter to use when looking up
authorities
- searchByBindPrincipal - whether to use username or bind principal
(user@domain)

Unit tests included.