[C++] Implement LogicalPlan and LogicalPlanBuilder

[asfimport]

Reporter: Francois Saint-Jacques / @fsaintjacques

Related issues:

• [Python] Make dataset Expression objects serializable (blocks)
• [C++][Compute] Add ExecNode hierarchy (is superceded by)

PRs and other links:

• GitHub Pull Request #6506

_{Note: This issue was originally created as ARROW-7878. Please see the migration documentation for further details.}