Adds minimum common shared code for custom actions API
This patch includes the initial data types, serialization,
string utilities, base exception classes and defines a
base action that includes a context argument.
The mistral_lib.api namespace was also removed in favor of
following the python convention of using an underscore to
denote private methods and considering all other methods
This patch removes code previously added as a placeholder
to match the spec. As of the Atlanta PTG, the purpose of
this library is now to store common code used within Mistral
ecosystem and 3rd party integration interfaces including
the Custom Actions API and Custom YAQL Functions API.
Partially-Implements: blueprint mistral-custom-actions-api
Partially-Implements: blueprint mistral-actions-api-main-entities