admin: streaming C++ API

[jmarantz]

Currently the admin API is designed so the implementation must collect and return the entire response in one contiguous block. This means that very long responses such as /stats and /config_dump may introduce significant delay and memory pressure.

Specificallly:

https://github.com/envoyproxy/envoy/blob/7bd0c0f4fe6b30e88a77c5739a70a78bc7400ef8/source/exe/main_common.h#L52

https://github.com/envoyproxy/envoy/blob/7bd0c0f4fe6b30e88a77c5739a70a78bc7400ef8/envoy/server/admin.h#L254

These two interfaces should be changed so they allow for streaming. Note that we have a dependence on the current non-streaming versions so we might need to have both old and new versions at least temporarily.

This is related to https://github.com/envoyproxy/envoy/issues/31082

[jmarantz]

I have a prototype of this interface locally. Want to get some internal review then I'll publish a PR.

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