Move `BodyParameter` definition to Azure.Core templates

[iscai-msft]

Clear and concise description of the problem

To clearly describe spreading behavior, we are vendoring template BodyParameter in specs if we don't want spread body parameters. We want to eventually move this to Azure.Core so we don't need to vendor. The options are either adding the template itself, like

model BodyParameter<T, name extends string = "body"> {
  @friendlyName(name)
  @bodyRoot body: T;
}

OR, by introducing a named parameter to templates, so spec authors would write this:

op doSomething<Something, BodyParameter=SomeParameters>

Checklist

• [X] Follow our Code of Conduct
• [X] Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[allenjzhang]

Captured in 1716 I prefer operation parameter override as it is widely used in ARM and is working well.