Info Endpoint

Mistral should have an optional endpoint, that could show useful information about current Mistral build.

Mistral has a lot of useful information that could be different from one Mistral build to another, just a few examples:
* mistral version
* versions of some custom actions
* some important config key-value
* other imaginable extensions
This data could be helpful for Mistral support team, which is not always the same as Mistral admin.
For this needs we can create new /info endpoint that will be optional and will show some prepared by Mistral admin data.

Proposed implementation:
For example, we want to show current mistral version via this endpoint.
For this, we should create a file in json format (e.g. /home/mistral_info.json):
{
    "version": "1.2.3.4"
}
After this, we should configure mistral:
[api]
enable_info_endpoint=True
info_json_file_path= /home/mistral_info.json

This configuration, which should disabled by default, turn on mistral-url/info endpoint.
This endpoint should handle GET requests and return json from this file.

This feature is generic, could be used for different goals and will be helpful.