Define metadata service API

[zhenghuiwang]

For v0.6 CUJ, we need to define metadata service API:

1. It should provide endpoints to create, read, update and delete metadata.
2. It should be able to connect related metadata in a machine learning workflow.
3. A UI for the CUJ can be built on top of this API and the built-in metadata(Define a set of metadata specifications for v0.6 #4),
4. It can handle customized metadata in a similar way to built-in metadata.

[zhenghuiwang]

/cc @neuromage @jlewi @mvartakAtVerta @terrytangyuan

[jlewi]

@zhenghuiwang How's this coming?

[zhenghuiwang]

@neuromage and I should be able to send out a proposal this week.

[zhenghuiwang]

#24 adds initial API and data model definitions

[jlewi]

@zhenghuiwang What is the remaining work to be done on this issue?

[zhenghuiwang]

An update on 4 items in this issue:

1. It should provide endpoints to create, read, update and delete metadata.

It is almost done. We have CRUD endpoints for artifact, artifact type, execution and execution types.
Some API changes for artifact proto definition need to do.

2. It should be able to connect related metadata in a machine learning workflow.

This is a stretch goal for logging the input/output relation between artifact and its execution. We may finish it later for artifact tracking with SDK and UI after this release.

3. The CUJ can be built on top of this API and the built-in metadata(Define a set of metadata specifications for v0.6 #4),

It is done. Built-in metadata definitions for this CUJ are merged and can be loaded during service start.

4.It can handle customized metadata in a similar way to built-in metadata.

It is done. Built-in metadata are registered by the artifact type and execution type endpoints.

[zhenghuiwang]

lineage tracking API is also added to the backend.