[ENG-1060] credit based trial support

[pjain1]

New flow -

1. Have two plans - one trial and one paid. Both will have same billable metric (slots/min or slots/hr). No need to add credits to plan, two plans are there for distinction so its easy to know which customer is on trial.
2. On new org creation - we create customer in Orb and Stripe as usual and also add $250 of credits directly to Orb customer so it persists across plan changes.
3. When credit drops below $50 we send out warning email
4. When credits deplete we hibernate projects, cancel subscription and send out email
5. On upgrade before or after credit depletion we move them to paid plan

Points to note -

1. All new trials will be credit based, no need to support time based trial
2. For customers currently on time based trials, they will also upgrade to new paid plan
3. Can remove extending time based trial feature for simplicity

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[begelundmuller]

LGTM