database/ldb29: LDAP-Like embedded database

ldb is a LDAP-like embedded database.

ldb provides a fast database with an LDAP-like API designed to be used
within an application.

In some ways it can be seen as a intermediate solution between key-value
pair databases and a real LDAP database.

This port is needed for Samba 4.20.

This commit is just a repocopy. I will update the port to the correct
version and connect it to the build in the next commit.

PR: 280533
Sponsored by: Klara, Inc.
Approved by: 0mp (mentor)
Approved by: samba (0mp, kiwi)
Differential Revision: https://reviews.freebsd.org/D48656