feat: add Plivo SMS provider

[sarveshpatil-plivo]

Adds a Plivo notification provider so Keep can send SMS alerts when a monitor fires, following the existing notification-provider pattern and sending through the Plivo Messages API.

Closes #6624.

Included

• keep/providers/plivo_provider/plivo_provider.py — PlivoProvider(BaseProvider) with a send_sms scope; _notify sends via messages.create(src, dst, text).
• Auth config: Auth ID, Auth Token (sensitive), source number / sender ID.
• Docs page + nav entry + autogenerated snippet, provider icon, and the plivo dependency (with the poetry.lock update).

Notes

• Scope validation uses a read (messages.list(limit=1)) instead of a test send — a Plivo send is billed and not idempotent.
• Auto-registers via the providers factory (no factory edit needed).
• Supersedes the earlier community PR feat: add Plivo SMS provider #5768 (closed with an unsigned CLA); the CLA will be signed here.

[CLAassistant]

All committers have signed the CLA.

[sarveshpatil-plivo]

Hi @talboren (cc @shahargl ), following up here.
This PR adds a Plivo SMS provider and the CLA is signed. It's a clean redo of the earlier community attempt (#5768 ) that was closed for an unsigned CLA.
Could you tell me if anything's missing or should something be changed?
Happy to make any changes required.