[ncp] add initial support of NCP

[Irving-cl]

This PR adds initial support of NCP in otbr-agent. When otbr-agent
starts, it will detect the Co-processor type. If the Co-processor is
NCP, otbr-agent will still run. But at this moment, it will provide
a dbus API to get the device role of the NCP, which is 'Disabled'
because no other functions are supported now.

This PR implements NcpSpinel which is the main class to communicate
with the NCP. Different from RadioSpinel in openthread, NcpSpinel
is totally async. There are no blocking operations like WaitResponse
in RadioSpinel.

Currently when the Co-processor type is NCP, most modules in otbr-agent
will be initialized and run. These may be added gradually later. And
the dbus server also initiates differently when Co-processor type is NCP.
When it's NCP, the dbus server registers AsyncGetPropertyHandler
instead of GetPropertyHandler used now.

Depends-on openthread/openthread#10272

[Irving-cl]

@bukepo @zhanglongxia @abtink @superwhd @wgtdkp

I think the change is a non-trivial one. The PR is still under development. There are many things to do: adding test, refactoring. But I hope that I can get some feedback on the framework and the direction in early stage. PTAL when you have a moment.

[Irving-cl]

This PR is ready for review now. It seems to be difficult to break
the PR down into smaller PRs. Here is an abstraction of what the PR
does:

1. Defined an interface (IControllerOpenThread) where we place the
   unified thread control APIs (for both NCP and RCP cases). In this PR,
   only one API (GetDeviceRole) is added for testing.
2. The original core class ControllerOpenThread is renamed as ControllerOpenThreadRcp
   and refactor'ed as an implementation class of IControllerOpenThread.
   Most of its original methods still exist.
3. Another class ControllerOpenThreadNcp is added to implement IControllerOpenThread
   and is used under NCP case.
4. The dependencies on ControllerOpenThread of the current modules
   are not changed at this staged. For example, BackboneAgent, NdProxyManager,
   BorderAgent all depend on an instance of ControllerOpenThread.
   Now they still depend on ControllerOpenThreadRcp, instead of IControllerOpenThread.
   Currently these modules won't be initialized and started under NCP
   mode. We may change this when later working on enabling these modules
   under NCP mode.
5. Refactor'ed the initialization process. Currently the modules hold
   a reference to ControllerOpenThread. And the reference is passed
   in the class constructors. In this PR, th reference is changed to
   pointer and pointer is passed in Init method because the instance
   is created later based on the co-processor type.
6. Renamed the variable Ncp. Currently the variable name for an
   instance of ControllerOpenThread is mNcp or aNcp. This is
   confusing from my perspective. The PR changes all these names to
   Ctrlr (Controller).
7. Added a test case to test the dbus API under NCP mode.

There is still a pending PR in openthread. Without it the CI here
fails. Please take a loook when you have a chance.
@zhanglongxia @abtink @superwhd

[Irving-cl]

Per discussion, the PR is still too large for review. I'll try to breakdown it.

The first breakdown PR is: #2294

Please review the simple one first. Thanks!