Add connection explorer

[polyaxon-team]

Feature description

Connections must be first promoted to the same level as projects/components/models.

Each connection should provide additional information and metadata about the underlying artifacts/assets. The new view should have the following use cases:

• Display meta-data created by users
• Display meta-data that could be generated automatically using a data-profiling or data-quality library
• Allow exploration of assets / artifacts, e.g. files in a dataset, containers of a registry, or tables/columns of a sql connection...
• Provide management and access control as well as tracking usage in the platform (jobs using the connections, frequent users, ...)