feat: add a private enterprise SD-ELEMENT for the log pipeline

[DavidCozens]

Write a private enterprise SD-ELEMENT. RFC 5424 reserves this form for definitions of your own, and
SyslogPipelineSd.c is a complete example of one: it implements the library's structured-data
extension point in its own translation unit.

static void SyslogPipelineSd_Format(struct SolidSyslogStructuredData* base, struct SolidSyslogSdElement* element)
{
    (void) base;

    SolidSyslogSdElement_Begin(element, "logPipeline", SYSLOG_ENTERPRISE_NUMBER);
    SolidSyslogSdValue_String(SolidSyslogSdElement_Param(element, "transport"), "tls");
    SolidSyslogSdValue_String(SolidSyslogSdElement_Param(element, "atRest"), "hmac-sha256");
    SolidSyslogSdElement_End(element);
}

static struct SolidSyslogStructuredData s_pipelineSd = {SyslogPipelineSd_Format};

... [logPipeline@32473 transport="tls" atRest="hmac-sha256"] device started

The vtable has one entry, Format, and the library never allocates the object. A stateless source
therefore needs no _Create and no pool slot; it is a static this application owns and points the
config at. A source with per-instance state puts that state alongside the vtable in the same struct
and reads it back from the base parameter.

A non-zero enterprise number is what produces a private SD-ID: _Begin emits name@number for one
and a bare IANA name for zero. SyslogEnterprise.h now defines the number and derives the string
that origin's enterpriseId carries, so the two forms cannot drift.

What the element reports is the state of the logging path. A collector can confirm that a record
arrived over TLS and was sealed at rest, and can alert on a device whose pipeline has weakened.
The remaining stages change both values as the protection changes.

When you need it. If a collector has to verify the protection a record travelled and rested
under rather than assume it.

[DavidCozens]

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