feat(models): Make assets searchable by their external URLs

[jjoyce0510]

Summary

In this PR we add the https://github.com/searchable annotation to the externalUrl field shared across entities! This enables us to find unique assets based on their unique URL, for use within the Chrome extension :)

Tested using this search input

{
 "input": {"types": ["DATASET"], "query": "*", "start": 0, "count": 10, "orFilters": [ { "and": [ { "field": "externalUrl", "values": ["https://www.google.com/"] } ] } ] }
}

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[chriscollins3456]

let's party