Add Signal Messenger integration (#28537)

* added signalmessenger integration

* allows to send a message (with an attachment) to one or more
  recipients

* added signalmessenger documentation to manifest file

* remove debug logging from signalmessenger integration

* add signalmessenger to .coveragerc

* fixed typo in signalmessenger manifes

* moved service specific code to own pypi library

* updated pysignalclirestapi dependeny in manifest.json

* added pysignalclirestapi requirement for signalmessenger component

* fixed typo in codeowners

* reworked signalmessenger integration based on code review input

* updated requirements for signalmessenger

* small code improvements in signalmessenger integration

* no need to use the get() method to access dict parameters that are
required

* small changes in signalmessenger integration

* re-ordered import statements
* removed empty "requirements" list (not needed)

* changed import order in signalmessenger integration according to PEP 8

* used isort to order includes in signalmessenger integration

* renamed signalmessenger to signal_messenger

* renamed signalmessenger to signal_messenger in CODEOWNERS file

* changed documentation url in signal_messenger integration to new name

* changed signal messenger naming in .coveragerc