Summary:
Add flag to disable analytics.
Change env prefix to be `PX` by default.
Support `PL` for backwards compatibility.
Parse flags in `init` so that we have access to flags before `Execute` (this
is necessary because `px.go` sets up the analytics client and enqueues events
before `Execute` is called.
Cleanup testing banner.
Cleanup flag overrides.

Test Plan:
`bazel build` cli and run a variety of commands with console
logging for sanity checks.

Reviewers: zasgar, michelle

Reviewed By: zasgar

JIRA Issues: PP-2624

Differential Revision: https://phab.corp.pixielabs.ai/D9116

GitOrigin-RevId: f52ac14f47dc0c5014bf4d9478ca7784f817c23f