MINOR: spoe: Add internal sample fetch to retrieve the SPOE engine ID

MINOR: spoe: Add internal sample fetch to retrieve the SPOE engine ID

The internal sample fetch "spoe.engine-id" is added. It may be used to
retrieve the current engine identifier, but only if the client endpoint is
an SPOE applet. For now, this sample is not documented. It will only be used
to set the connection pool name for a specific engine. This way, several
engine can use the same SPOP backend without sharing their idle connections.

The documentation will be added later, mainly because other SPOE sample
fetches will be added, and some changes are expected.

The related issue is #2502.