Search handler and client with HTTP POST-based request serialization.

[isoos]

• Display the /my-liked-packages page the same way as other package listing. #8887
• moved TextMatchExtent to pkg/_pub_shared, and added the new data class there, as the build command's inclusion rules otherwise couldn't identify all the required types.
• Updated handler creation and processing, but kept both search variants on /search, since we have so little change based on the HTTP method
• Update the search client to call the POST only when the new experimental flag is enabled.
• Restructured the retry-GET handler and created withRetryHttpClient to reuse some of its logic
• Note: we need to have a better search isolate/handler integration test, but that is independent of this change, tracking in Create better integration test(s) for search isolate + search handler + search client. #8891