What is the purpose of the change

(For example: AppConn Core defines the abstractions and interfaces of the AppConn core functions.
It is the core concept that enables DSS to easily and quickly integrate various upper-layer web systems.
Related issues: #590. )

Brief change log

(for example:)

• Define the core abstraction and interfaces of the AppConn;
• Define the core abstraction and interfaces of the three-level specifications of AppConn.

Verifying this change

(Please pick either of the following options)
This change is a trivial rework / code cleanup without any test coverage.
(or)
This change is already covered by existing tests, such as (please describe tests).
(or)
This change added tests and can be verified as follows:
(example:)

• Added tests for creating and execute the workflow contains the nodes that DSS has integrated to go through and verify the availability of different AppConns.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• Anything that affects deployment: (yes / no / don't know)
• The Core framework, i.e., AppConn, Orchestrator, ApiService.: (yes / no)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)