Fix ExtendedRequest requestValue RFC4511 encoding

[michele-deluca]

Problem
RFC 4511 defines ExtendedRequest as:

ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
requestName [0] LDAPOID,
requestValue [1] OCTET STRING OPTIONAL
}

The current implementation appends er.Value directly as a child of the ExtendedRequest:

if er.Value != nil {
pkt.AppendChild(er.Value)
}

ExtendedRequest
├─ requestName [0]
└─ SEQUENCE {...}

ExtendedRequest
├─ requestName [0]
└─ requestValue [1] OCTET STRING

This breaks interoperability with LDAP extensions expecting RFC-compliant encoding.

One example is the eDirectory NMAS Universal Password extension (OID 2.16.840.1.113719.1.39.42.100.13), which returns:

NMAS Extension invalid version

with the current implementation.

Fix

Serialize er.Value and encode it as the context-specific [1] requestValue field defined by RFC 4511.

Validation

Validated against eDirectory/OpenLDAP interoperability tests.

Before patch:

NMAS Extension invalid version

SEQUENCE {
INTEGER 1
INTEGER 0
OCTET STRING ""
}

[cpuschma]

Fixed in #628 with tests. Needs overall work done.