Certification of Data Entities

[etr2460]

Is your feature request related to a problem? Please describe.
In larger deployments of Superset, it can be difficult to know which tables, datasources, and metrics are the correct ones to use. Many entities may have similar names, but no structured way exists to find the single source of truth.

Describe the solution you'd like
I propose the addition of a certified field on various data entities (tables and sql_metrics to start). Certification represents the review of a data entity by someone (a person, a group, a process) which adds an additional level of trust and verification to the entity.

This field will live JSON encoded in a new extra column added to the metadata database for these two tables. The structure of the data will be as follows:

{
  "certification": {
    "certified_by": "Erik Ritter",
    "details": "This metric is the single source of truth for births." 
  }
}

These certifications will be exposed in the UI anywhere a table or metric is referenced with the use of an icon before the table/metric name. In the future, we can also extend this approach to other data entities such as columns, slices, and dashboards.

Describe alternatives you've considered
I'm proposing a generic extra column as opposed to a more structured certifications table because I anticipate some iteration on this feature, and using a generic JSON string column allows for faster and safer iteration, as well as reduces the number of db migrations required. After all work concludes and the product is in a stable place, we can consider a migration to a more structured db representation.

Additional context
Tableau's implementation of certification

I'll be PR-ing out work around this feature in the next couple weeks, so here's the context behind the what/why of these PRs!

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[etr2460]

Closing this issue since a lot of the work is done already and it's tracked in the roadmap: apache-superset/superset-roadmap#73