[Telemetry] Move converters into separate file

Move ConvertMojoRoutine and ConvertRoutineStatus methods into a separate
converter file: diagnostics_api_converters.h/cc.

Bug: b:188897824
Test: added unit tests in diagnostics_api_converters_unittest.cc
Test: out/Default/unit_tests
--gtest_filter="*TelemetryExtensionDiagnosticsApiConverters*" [Green]
Test: out/Default/browser_tests
--gtest_filter="*TelemetryExtension*ApiBrowserTest* [Green]

Change-Id: I311d803ff964b5316278c034125e36ec436968e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3144170


Commit-Queue: Mahmoud Gawad <mgawad@google.com>
Reviewed-by: Oleh Lamzin <lamzin@google.com>
Cr-Commit-Position: refs/heads/main@{#918773}