[AIRFLOW-4417] Add AWS IAM authenication for PostgresHook

[andrewhharmon]

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-4417

Description

• Enhance the existing PostgresHook to allow for IAM authentication for RDS Postgres and Redshift.

Tests

• My PR adds the following unit tests
  • test_get_conn()
  • test_get_conn_rds_iam_postgres()
  • test_get_conn_rds_iam_redshift

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #5223 into master will increase coverage by 0.01%.
The diff coverage is 95%.

@@            Coverage Diff             @@
##           master    #5223      +/-   ##
==========================================
+ Coverage   78.55%   78.56%   +0.01%     
==========================================
  Files         469      469              
  Lines       29983    30003      +20     
==========================================
+ Hits        23552    23571      +19     
- Misses       6431     6432       +1

Impacted Files	Coverage Δ
airflow/hooks/postgres_hook.py	96.42% <95%> (+1.98%)	⬆️
airflow/models/taskinstance.py	92.42% <0%> (-0.18%)	⬇️

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[ashb]

Nice

[mik-laj]

Can you also update the docs file?
https://github.com/apache/airflow/blob/master/docs/howto/connection/postgres.rst

[andrewhharmon]

@mik-laj whats the best way to do that? will i just need to create a new PR? Can it reference the same JIRA?

[mik-laj]

@mik-laj You do not need to refer to the Jira ticket. Changes to the documentation can be made using [AIRFLOW-XXX]. If you want to make a change then you need to create a new PR, because it has already been closed.