feat: PesaPal payment integration

[AbogeJr]

Brief summary of the change made

Adds a PesaPal payment provider plugin so mini-grid operators in East Africa (Kenya, Uganda, Tanzania) can accept hosted-checkout payments via PesaPal v3

docs/integrations-guide/pesapal-integration.md.

closes: #1044

Are there any other side effects of this change that we should be aware of?

Describe how you tested your changes?

1. set up two cloudflared quick tunnels (frontend 8001, backend 8000) with APP_URL and
   MPM_BACKEND_URL pointed at the backend tunnel
2. Enable the plugin via Settings
3. Paste demo consumer_key / consumer_secret as provided on https://developer.pesapal.com/api3-demo-keys.txt, save. Backend calls RegisterIPN against /api/pesapal/ipn/{companyId}; the response populates ipn_id and ipn_registered_at on the credential.
4. Open the permanent payment URL from /pesapal/overview in a new window.
5. Submit a known meter serial + KES 10. Browser is redirected to PesaPal hosted checkout, payment completed with M-Pesa.
6. PesaPal redirects the customer back to /#/pesapal/public/result/...

Pull Request checklist

Please confirm you have completed any of the necessary steps below.

• Meaningful Pull Request title and description
• Changes tested as described above
• Added appropriate documentation for the change.
• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[beesaferoot]

@AbogeJr, good work with the PR. I followed the steps and was able to test payment failure flow (because I don't have an active MPesa Number)

I noted some minor improvements below.

[beesaferoot]

Other minor fixes to be made.

UI aligment

The warning message should be properly aligned.

Failed transactions still has status as "requested"

Failed transactions should correctly trigger the right failure routines that update the transaction status.

[AbogeJr]

@beesaferoot
Noted. Thanks for the quick review. I'll ping you as soon as the changes are up

[AbogeJr]

@beesaferoot
I have addressed the changes requested. Please let me know once you check it out.

Tested on Live as well.

[beesaferoot]

Looks good to me, thanks @AbogeJr

@dmohns Do you have any comments to add?

[dmohns]

Looks really, really good. Thanks a lot @AbogeJr 🙏

I left a few small, nit comments. Currently, the PesaPal account creation seems broken, so I cannot test end2end. Leaving the approval for @beesaferoot

[beesaferoot]

@AbogeJr Can you update your branch?

[AbogeJr]

@beesaferoot
Done