Issue 5989 - RFE support of inChain Matching Rule

Bug description:
	Computation of membership (like 'memberof') is a common issue.
	The issue is more expensive to solve when there are nested membership.
	For example "gives me all the groups this entry belongs to" or "gives me
	all subordinates having this manager".
	Either the LDAP client computes the values or dedicated plugin (like 'memberof')
	maintains direct membership attribute for the LDAP client.
	InChain Matching Rule allow a LDAP client to request the server to compute this membership.

Fix description:
	The implementation is designed https://www.port389.org/docs/389ds/design/matching-rule-in-chain.html

relates: #5989

Reviewed by: William Brown, Mark Reynolds, Pierre Rogier (Thanks !)

Makefile.am

@@ -1738,6 +1738,7 @@ libsyntax_plugin_la_SOURCES = ldap/servers/plugins/syntaxes/bin.c \
 	ldap/servers/plugins/syntaxes/facsimile.c \
 	ldap/servers/plugins/syntaxes/guide.c \
 	ldap/servers/plugins/syntaxes/int.c \
+	ldap/servers/plugins/syntaxes/inchain.c \
 	ldap/servers/plugins/syntaxes/nameoptuid.c \
 	ldap/servers/plugins/syntaxes/numericstring.c \
 	ldap/servers/plugins/syntaxes/phonetic.c \