b/457332685

This simulates the Json and gRPC responses for object contexts, which
will be helpful in gcp c++ integration tests in gcp c++ feature implementation:

Based on responses observed from real backend and API definition,

Insert or Complete Update
Input: {contexts: {custom: {key: value}}}
Update_mask: contexts
Simulation: Add new entries to the object.contexts and furnish with create_time and update_time

Patch (Update an existing key)
Input: {contexts: {custom: {key: new-value}}}
Update_mask: contexts.custom.key
Simulation: Update the value of the key to "new-value" and furnish with update_time

Patch (Delete an existing key)
Input: {contexts: {custom: {key: null}}}
Update_mask: contexts.custom.key
Simulation: Delete that entry with that key from object.contexts

Patch (Delete all the contexts)
Input: {contexts: {custom: null}}
Update_mask: contexts:custom
Simulation: Delete the contexts field from object, making that object looks like its default