Add single value enum when using const keyword for compatiblity

[uasouz]

Use extra single value enum where const is used due to lack of support from jsonschema2pojo

Many thanks for submitting your Pull Request ❤️!

What this PR does / why we need it:
jsonschema2pojo does not support const keyword yet.
This was causing a bug when using resusable tasks that have no value on with block. It would always match to CallAsyncAPI.

In order to achieve proper behaviour I added a single value enum below each const. This will make it compatible with jsonschema2pojo while they solve the issue #1001.

joelittlejohn/jsonschema2pojo#1001.

Also addded a test to validate the behaviour

Special notes for reviewers:
I think this may be a good idead to adopt as a single enum on the main spec too. const and single value enum are the same thing under the hood, for jsonschema2pojo at least.

Additional information (if needed):

[fjtirado]

@uasouz I think we should add support to const.
To implement this workaround, we should change the original schema in the spec repo (the main target is for them to not differ in a single comma, actually copying the schema was a workaroung till pojo2scheme add support for non local file paths)

[uasouz]

@fjtirado 100% agree, we should.
I will then open another PR on the main specification repository for the workaround.
So if it is ok, you can close this PR.

[fjtirado]

@uasouz Actually I believe we should use another PR to add support to const. The idea will be to incorporate to our serializer the const check. so, if string is different from http, HttpTask deserializer will fail. I can work on that, so I will open the issue and the PR, you do not need to change the origina schema.

[fjtirado]

@ricardozanini Please close this issue and assign #418 to me