feat(cicd): add conditional field in pipeline manifest for connection_name

[huanjani]

These changes add a commented-out optional field in the pipeline manifest for connection_name when the source provider is GitHub or Bitbucket.
Manually tested with non-CSC, Bb, GH, CSC w/ existing connection, CSC w/o existing connection.

Updated docs accordingly.

Related #2042, Resolves #2012, Resolves #1247, Related #1339.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[huanjani]

Non-CSC:

  # Additional properties that further specifies the exact location
  # the artifacts should be sourced from. For example, the GitHub provider
  # has the following properties: repository, branch.
  properties:
    branch: master
    repository: https://us-west-2.console.aws.amazon.com/codesuite/codecommit/repositories/aws-sample/browse

# The deployment section defines the order the pipeline will deploy
# to your environments.

CSC:

 # Additional properties that further specifies the exact location
  # the artifacts should be sourced from. For example, the GitHub provider
  # has the following properties: repository, branch.
  properties:
    branch: master
    repository: https://huanjani@bitbucket.org/huanjani/aws-copilot-sample-service
    # Optional: specify the name of an existing CodeStar Connections connection.
    # connection_name:

# The deployment section defines the order the pipeline will deploy
# to your environments.

[efekarakus]

Code looks good, just some documentation updates 🙏

[bvtujo]

Looks good!