expose DBus interface method names as symbols

Enable a way to access the method names of a given DBus interface as a
constexpr symbol via the header.

4 use-cases:

- printing error / debug logs wit

expose DBus interface method names as symbols

Enable a way to access the method names of a given DBus interface as a
constexpr symbol via the header.

4 use-cases:

- printing error / debug logs with the method name.
e.g. 'error calling ${METHOD_NAME}'

- accessing DBus methods in applications which do not yet use the PDI
generated bindings.

- preventing typos in DBus method names

- estimating the impact of removing a given DBus method from an
interface. When using these symbols, it would cause a build failure in
applications relying on the existence of that method.

This change is similar to [1] and goes into the same direction.

Tested: Newly written unit test passes.

References:
[1] d2571922bfdb4f6b41ba4fbc45b8a4272793fd40

Change-Id: Id423c3a668dd1a8346040f4380476d5e02468ddb
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>

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