tsp, internal model generate to implementation package

[weidongxu-microsoft]

link #2286

main code

• 82fa9f3 for internal model
• a992538 for internal API (currently only cover the models used -- any model used by internal API is treated as internal model)
• 241890a for rename SchemaContext.ConvenienceApi to SchemaContext.Public

model/class would have either Public or Internal (and optionally External for class from other lib), but not both.

API could have ConvenienceAPI and Internal (currently if Internal, it is not ConvenienceAPI -- this can change when we decide to generate "package private" Convenience method for Internal).

[weidongxu-microsoft]

openai need to have change to client.tsp to move some internal to public

1. remove them from "implementation.models" (as they currently in "models")
2. keep them in sync with TypeSpec models

@@access(Azure.OpenAI.FunctionName, Access.public, "java");
@@access(Azure.OpenAI.AzureChatExtensionConfiguration, Access.public, "java");
@@access(Azure.OpenAI.AzureChatExtensionsMessageContext, Access.public, "java");
@@access(Azure.OpenAI.ChatCompletions, Access.public, "java");
@@access(Azure.OpenAI.FunctionDefinition, Access.public, "java");
@@access(Azure.OpenAI.ImageSize, Access.public, "java");

result Azure/azure-sdk-for-java#36642

PS: there maybe more to add to Access.public, to help them get synced

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Also we probably no longer need these "custom-types" configure.

It should be covered by API marked as @@access(op, Access.internal) (which make all related class Access.internal as well, and by default in "implementation.models")

    custom-types-subpackage: "implementation.models"
    custom-types: "FunctionCallModelBase,FunctionCallPreset,FunctionCallPresetFunctionCallModel,FunctionNameFunctionCallModel"

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This be delayed to #2300