rename top_sha to "sha" and add final_sha to task-related APIs

To adapt to COIN-974, all rest APIs which return a task item should report the final_sha of the given task. This final_sha represents the new branch head after gerrit completes all necessary merges and cherry-picks for changes included in an integration.

To reduce confusion, the field "top_sha" should be renamed "sha". This field will still represent all changes included in an integration, but may not be representative of the final branch state after gerrit merges changes.

Suggested documentation notes:
sha: This sha includes all changes tested in an integration pre-merge. Use this field with caution, as this sha may not exist in the source branch following merge. See final_sha.

final_sha: This post-merge sha on the source branch includes all changes tested in an integration.