Implements a series of bugfixes and improvements to enable integration with WordPress Playground. Some highlights:

• Add blueprint.http_client filter so Playground can setup its own HTTP Client instance
• Add blueprint.resolved filter to enable injecting additional steps in Playground
• RequestReadStream doesn't expect Content-Length when Content-Encoding is present.
• WPCLIStep now gets an explicit --path argument to avoid relying on WordPress autodetection heuristics
• Restored 64KB as the default byte stream chunk size
• latest is now a valid WordPress version identifier
• Importing content is now supported even when running as phar
• StreamImporter now requires new_site_content_root_url and a few other options when they can't be backfilled via WordPress functions. For example, when WordPress hasn't yet been loaded
• Magic strings, such as create-new-site, are replaced with constants, such as self::EXECUTION_MODE_CREATE_NEW_SITE
• ...a lot more other tiny improvements.

Testing instructions

Confirm the CLI failures are in line with trunk (most successes, some flaky failures)