phosphor-logging extensions

[Matt Spinler]

Hi,

In the past I have written about phosphor-logging library plugins.
We've now morphed that into having logging extensions that live in the
phosphor-logging repo.  They would be completely compiled out and
disabled unless they were explicitly enabled.  The point of these is to
allow other log formats to be created without incurring extra D-Bus call
overhead.

Our use case is for a special Platform Event Log (PEL) format that we
have to support on the BMC.  We discussed (a lot) about having a
standalone PEL daemon, but the D-Bus overhead was deamed to be too
large.

I have a prototype of this up at
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-logging/+/2189.

I also discuss it in a design document that covers how to handle PEL
support at https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/21368.