Fix base URL handling

[asonnenschein]

Problem
Planet API base URLs are currently hardcoded in-line source code and point to production systems, which makes it difficult for internal contributors (like Planet employees) to test and run other procedures against environments other than production.

Solution
Make base URLs in the Python SDK configurable parameters, with defaults that point to production systems for users.

A/C

• CLI supports optional arguments for running commands against custom API base URLs. If no base URL is provided, then the CLI should default to production systems.
• SDK supports optional arguments for configuring to use custom API base URLs. If no base URL is provided, then the SDK should default to production systems.
• Update docs.