[Mgmt]Build enums from code model converter

[pshao25]

This PR is to gradually support code model conversion to input types with several PRs, instead of a huge PR at one time.

Current behavior:
We have

1. Different kinds of schemas in code model. (and operations)
2. Constructors for these schemas as public TypeProvider(Schema schem){}

Final goal:
We want:

1. Schame mapping to input type pseudo as InputType inputType = CodeModelConverter.From(schema).
2. Constructors for these input types as public TypeProvider(InputType inputType){}

Solution:
We could support one kind of schema/input type pair at one time. For example, in this PR, it supports ChoiceSchema/InputEnumType. Therefore this PR would introduce (which actually already exists):

1. InputEnumType inputType = CodeModelConverter.From(ChoiceSchema/SealedChoiceSchema).
2. public EnumType(InputEnumType inputType){}

To support one type at one time, we might need to expose CodeModelConverter and the mappings it builds temporarily as what this PR did. In the transition period,

1. CodeModelConverter is exposed all the time.
2. We get type from input types if we support.
3. We get type from code model if we not support.

After we convert all the types to input types, we remove all the exposed things from CodeModelConverter.

[ArcturusZhang]

I think before we go any further, we should remove this MgmtContext first to make everything explicit.
Also we should remove BuildContext.