feat(sdk): expose IfPresentPlaceholder and ConcatPlaceholder to kfp.dsl

[zichuan-scott-xu]

Description of your changes:
This PR exposes IfPresentPlaceholder and ConcatPlaceholder to kfp.dsl and add examples to show how they could be used to author containerized components.

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[google-oss-prow]

Hi @zichuan-scott-xu. Thanks for your PR.

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[connor-mccarthy]

@zichuan-scott-xu, this looks really good. I'm curious if we can find a way to improve the way children of BaseModel are documented by our reference docs so that only the official user-facing methods and attributes are documented. Currently, methods like split_cel_concat_string and to_placeholder_string are getting documented. We don't want to promise the stability of anything other than the constructor API if we can help it. Plus, these helper methods add clutter to the docs.

Currently we use Sphinx automodule to automatically document the .dsl module. The source .rst file that specifies how to do this is here.

If you cd docs then run make clean html, then open _build/html/index.html, you can build and inspect the docs from source.

An easy approach to handling this would be to use an _ prefix to make the method hidden, but this involves changing quite a bit of code, down to the BaseModel methods. This is fine if we want to go this route, but I'm curious what you think and if there are any other approaches you like more.

[connor-mccarthy]

/ok-to-test

[connor-mccarthy]

Let's handle this documentation change in a separate PR.

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: connor-mccarthy

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[zichuan-scott-xu]

/retest

[zichuan-scott-xu]

/retest

[zichuan-scott-xu]

/retest