[WIP][linux] add ThreadStackManager for Linux Device layer

[gjc13]

Problem

This PR adds ThreadStack manager in the linux device layer.
It relies on otbr-agent as the Thread daemon.

Summary of Changes

• Implementation of ThreadStackManager
• DBus connection driver in the event loop
• Functional tests for ThreadStackManager

fixes #740

[andy31415]

Could you update a description of what a ThreadStackManager is ?

I see this adding DBus support and I am aware what Thread is, but am missing context of what ThreadStackManager is and what is being built. A brief blurb in the PR would be nice.

Should the title still be WIP or is this ready for rewview and merge?

[andy31415]

should we remove commented out code?

is there any way for this test to fail besides a SIGSEGV? I see it initializing a lot of things but never check any error codes. Can we make it a unit test instead or provide some instructions on how run the test?

[gjc13]

Could you update a description of what a ThreadStackManager is ?

I see this adding DBus support and I am aware what Thread is, but am missing context of what ThreadStackManager is and what is being built. A brief blurb in the PR would be nice.

Should the title still be WIP or is this ready for rewview and merge?

The ThreadStackManager implementation itself is completed. I open this PR to gather feedback from the community about the Thread solution. Does it look good to use otbr-agent as the Thread daemon and DBus as the IPC mechanism on Linux? I also open it for CI verification and there's still some work to do in the functional tests.

The ThreadStackManager is required for the chip stack to set the network masterkey and other network arguments so as to join a Thread network.

One of the use case is the setup process. User will pass the Thread network information from the phone to a setup server in the chip daemon and it will in turn be passed to _SetThreadProvision in the ThreadStackManager.