Commit Message: In #19693 a streaming stats implementation was introduced, dramatically reducing the amount of time and memory it takes to generate an admin http response. /config_dump (#32054) and /clusters_ (#32054) can also be improved in this way.

However, in some environments we do not want to expose an HTTP port for admin requests, and we must use the C++ API. However that API is not streaming: it buffers the entire content. This PR adds a new streaming API.

Mechanically this new functionality was easy to add as an externally callble C++ API as the Admin class already supported this model on behalf of /stats.

However there's a fair amount of complexity managing potential races between active streaming requests, server exit, and explicit cancellation of an in-progress request. So much of the effort and complexity in this PR is due to making that thread-safe and testing it.

Additional Description:

Note to reviewers: if this PR is too big it can be broken into 2. The significant additions to main_common.cc, main_common.h, and main_common_test.cc need to stay together in one PR, the rest of the changes are non-functional refactors which are needed for the larger changes to work; they could be reviewed and merged first.

Risk Level: medium -- this is a new API but it does refactor some existing functionality. Using the new functionality will add some risk also as there is some careful thought required to believe we have protected against all possibilities of shutdown/cancel/admin-request races.
Testing: //test/..., and lots of tsan repeated tests for the new streaming tests. Lots of iteration to ensure every new line of main_common.cc is hit by coverage tests.
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a
Fixes: #31755

Request variables

Key	Value
ref	5979107d40770cba652d4f53861057da487c6c58
sha	4fa21ea
pr	32346
base-sha	8318716
actor	@jmarantz
message	admin: add streaming variant of the admin API...
started	1709832711.180559
target-branch	main
trusted	false

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