[rllib] Port DDPG to the build_tf_policy pattern

[ericl]

What do these changes do?

This ports DDPG to the policy builder pattern. This is the last major algorithm that needed to be ported.

Pendulum performance seems to be on par. @joneswong could you check if parameter noise exploration still works as expected? There was a lot of changes around handling in that code.

fyi @qxcv @gehring

Related issue number

Closes #4822
Closes #4788

Linter

• I've run scripts/format.sh to lint the changes in this PR.

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