cpp: add template_streamop generation with runtime/compiler test coverage

[zsy056]

Add template_streamop support in the C++ generator so generated operator<< and printTo can target generic stream-like output types. Keep default behavior unchanged when the option is not set (std::ostream signatures remain). Add compiler/runtime coverage for template generation, friend declaration correctness, enums, and collection printing.

default:

std::ostream& operator<<(std::ostream& out, const SimpleStruct& obj);

class SimpleStruct {
public:
  void printTo(std::ostream& out) const;
};

with template_streamop:

template <typename OStream_>
OStream_& operator<<(OStream_& out, const SimpleStruct& obj);

class SimpleStruct {
public:
  template <typename OStream_>
  void printTo(OStream_& out) const;
};

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.