Add support for Alexa.StepSpeaker

[lucasweb78]

Description:

Adds support for Alexa.StepSpeaker as per https://developer.amazon.com/docs/device-apis/alexa-stepspeaker.html

Maps entities that support SUPPORT_VOLUME_MUTE and SUPPORT_VOLUME_STEP to the Alexa Capability Alexa.StepSpeaker

Checklist:

• The code change is tested and works locally.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.