[codegen][Go] Fix compilation error of generated go code when schema is free form object

Created by: sebastien-rosset

codegen:

1. Add helper function isAnyTypeSchema()
2. Add code comments to clearly differentiate between free-form object (which must be a map of string to value) and any type value (which can be array, object, string, integer...).
3. Remove unused min/max fields in CodegenProperty
4. Add code comments in CodegenProperty
5. Fix equals() method in CodegenProperty. The deprecated attribute was a comparison to itself

golang: When a schema is Free Form object, the go-experimental generator sets the golang dataType to NullableMap[string]interface{}, which is not a valid go type. This causes a compilation failure of the generated code.

This PR is a fix for this problem.

components:
  schema:
        stuff:
            type: object
            nullable: true

Issue: #5387

From this PR, it's clear there is a different interpretation of the OAS spec between the go-experimental generator versus the python-experimental generator. This is a problem that needs to be addressed to get consistency across languages.

• In the generated go client, the only accepted type is a map of string to values
• In the generated python client, any valid JSON document will be accepted.

PR checklist

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