feat(ingestion): Enable lower-casing of the name part of dataset urn if env variable is set.

[rslanka]

This is an experimental temporary workaround for deployments starting fresh to get all dataset urns to use lower-casing for the names, if the environment variable DATAHUB_DATASET_URN_TO_LOWER is set to true either on the client or server side.
NOTE: Existing deployments would run into issues if this is set, before we ship a migration script that needs to run first.

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

[github-actions]

Unit Test Results (build & test)

  96 files  ±0    96 suites  ±0   25m 56s ⏱️ + 6m 15s
689 tests ±0  630 ✔️ ±0  59 💤 ±0  0 ❌ ±0 

Results for commit bcbc64a. ± Comparison against base commit 5b22d96.

[github-actions]

Unit Test Results (metadata ingestion)

       5 files  ±0         5 suites  ±0   1h 6m 6s ⏱️ + 10m 16s
   425 tests ±0     425 ✔️ ±  0    0 💤 ±  0  0 ❌ ±0 
2 050 runs  ±0  2 006 ✔️ +20  44 💤  - 20  0 ❌ ±0 

Results for commit bcbc64a. ± Comparison against base commit 5b22d96.