fix(ingest): redshift source gets external table types properly

[treff7es]

Earlier the redshift source failed to get varchar sizes and because of this all the varchars become NullType.
Here is the corresponding sqlalchemy code why this happened -> https://github.com/sqlalchemy-redshift/sqlalchemy-redshift/blob/75db33620b87e732386b1b63986b67d058689f6c/sqlalchemy_redshift/dialect.py#L694

I changed the query back to the sqlalchemy default (https://github.com/sqlalchemy-redshift/sqlalchemy-redshift/blob/75db33620b87e732386b1b63986b67d058689f6c/sqlalchemy_redshift/dialect.py#L777) and I unioned the external tables to it as well.
This way the varchar sizes showed up properly and even column encoding and dist/sort keys get populated.

The only caveat was that the external column types are different from the postgres types, so I had to map those to one of the redshift types. -> https://docs.aws.amazon.com/redshift/latest/dg/r_CREATE_EXTERNAL_TABLE.html (see datatypes part)

Here is a comparison how the columns looked like with the old query:

How it looks like with the new query:

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)

[github-actions]

Unit Test Results

  39 files  ±0    39 suites  ±0   18m 1s ⏱️ +5s
314 tests ±0  314 ✔️ ±0    0 💤 ±0  0 ❌ ±0 
963 runs  ±0  941 ✔️ ±0  22 💤 ±0  0 ❌ ±0 

Results for commit da8af55. ± Comparison against base commit 561c04b.

♻️ This comment has been updated with latest results.

[shirshanka]

LGTM!