feat(ingest): looker - reduce memory requirements

[shirshanka]

Changing the process to conserve memory.
Instead of caching dashboards, we cache the explores.

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

[treff7es]

lgtm

[github-actions]

Unit Test Results (metadata ingestion)

       8 files         8 suites   56m 6s ⏱️
   671 tests    668 ✔️ 3 💤 0 ❌
1 342 runs  1 336 ✔️ 6 💤 0 ❌

Results for commit b35be87.

[github-actions]

Unit Test Results (build & test)

517 tests  ±0   517 ✔️ ±0   9m 7s ⏱️ +48s
121 suites ±0       0 💤 ±0 
121 files   ±0       0 ❌ ±0 

Results for commit b35be87. ± Comparison against base commit 2a95248.