1. All greeter servers now support flag `--port` to customize the listen
port.
2. All client implementation now have `--target` flag to specify the
server location.
3. ABSL is used to parse the flags to reduce the amount of boilerplate
code in the examples.
Fixes: #26989
* De-experimentalize callback API
* Make FromServerContext based on ServerContextBase
* Fix lambda
* Fix headers
* De-experimentalize tests
* clang-format
* Fix consistency checks
* wip
* Fix const-ness of callback client read RPC requests
* Fix golden file
* Give full route_guide callback client example with Hold etc
* Complete example route-guide server
* De-experimentalize test services
* Omit unneeded using
* Remove some uses of non-experimental macro from test
* clang-format examples
* De-experimentalize async stub calls
* Remove experimental namespace use in qps, ubms
* De-experimentalize alarms, generic stubs, and context allocator
* De-experimentalize SetContextAllocator
* clang-format
* Fix conflicts
* Leave obsolete API in place until users can be migrated
