[Feature] Rename Workflow to Agent

[xintongsong]

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Description

We would like to rename the concept Workflow to Agent, for the following reasons:

• The project is framework for building agents, but we have not clearly defined what is an agent. In our design, an agent is equivalent to a workflow, that acts like a complex custom operator that consumes a datastream / table and produce another datastream / table in flink.
• A Flink job itself can be considered as a streaming workflow. If the agent built with flink-agents, which consists of a series of operators in the Flink job's topology, is also called a workflow, that could be confusing for the users.

Are you willing to submit a PR?

• I'm willing to submit a PR!

[xintongsong]

Updated design docs #9 and #41 accordingly.