Cert: Add @metadata decorator for cert tests

* Add @metadata decorator to dump metadata into UserData section of
  test_summary.yaml file after the test run.
* Metadata is recorded per test case as key-value pairs
* TEST_CLASS and TEST_NAME combination can be used to correlate a piece
  of metadata with the ACTS or Mobly generated test run record in the
  same YAML file.
* Metadata is only guaranteed to be written when the test result is PASS,
  FAIL or SKIPPED. When there are test infrastructural errors, metadata
  might not be written successfully.
* Added unit test to verify various usage of @metadata.
* Sample output:

Type: UserData
Test Name: test_basic_operation_request_connection
Test Class: L2capTest
PTS Test ID: L2CAP/COS/CED/BV-01-C
timestamp: 1585644882317

Test: source gd/cert/run --host
Bug: 152828141
Change-Id: Iff2ba7a9a572ad9496a5fb9142ff66f41a18cd9d