Process additionalProperties: true as arbitrary type instead of object

[impl]

When a schema specifies additionalProperties: true, we need not restrict those properties to a particular type. This change sets the schemas for them to AnyType instead of object.

From a generation perspective, this only changes the output for generators that differentiate between AnyType and object in their type mappings; most do not. This fixes at least one bug in the Go and TypeScript generators.

• Fixes [BUG] ModelUtils.getAdditionalProperties returns ObjectSchema for "true" #9282
• Fixes [BUG][GO] Invalid property type for free form object #3277 (Go-specific)

New tests for the Go generator and DefaultCodegen demonstrate the intent of this change.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

For Go, @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d
For TypeScript, @TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero

Thanks in advance for the review!

[wing328]

Tested locally and the result is good. Thanks for the PR

FYI @OpenAPITools/generator-core-team