[sdbusplus] To generate a common header for public information of interfaces

[Lei YU]

Currently, sdbus++ generates the server code separately, and in
phosphor-dbus-interfaces, the separated headers are installed, and the
separated cpp files are combined together, and compiled as a library.
The public information of the interfaces (e.g, the interface strings,
the enum strings) are either in the separated header or in the cpp
files.

The result is, when a phosphor service needs to use an interface, or
an enum, it has to define the interface string or the enum string
manually, and it happens everywhere.

How about
1. Making sdbusplus to generate a "common" header for each interface
including the public information;
2. Then phosphor-dbus-interfaces could generate a single header file
that includes all the public information of the interfaces;
3. Then the phosphor service could include a single header file, and
use the interface/enum strings it needs, without manually defining
them?

There is an initial concept implementation:
* https://github.com/mine260309/sdbusplus/commit/78cb63fb7e1ceb62165c71e15779f23f7e9f166c
* https://github.com/mine260309/phosphor-dbus-interfaces/commit/6079d25547f0143fc7562a1c7ee6beb888a66432

With the above changes, a service could directly use the generated
interface string, e.g.
`sdbusplus::xyz::openbmc_project::Software::ApplyTime::interface`.
In the future, the enum strings could be put there and thus we do not
have to manually write the long full qualified string.

Ideas and suggestions are welcome.