feat: Add Plivo as an SMS and voice alert-notification provider

**What would you like to be added**:

Plivo SMS and voice providers under `alert/sender/provider/`, so alerts can be delivered as a Plivo SMS or an outbound Plivo voice call, following the existing per-vendor provider pattern (Tencent, Aliyun).

Two providers: `plivo_sms_provider.go` (`plivo-sms`, `POST …/Message/`) and `plivo_voice_provider.go` (`plivo-voice`, `POST …/Call/`), registered in `init.go` with default templates in `models/message_tpl.go`.

They're config-driven like the existing providers (URL/method/body come from the notify-channel config). The code injects HTTP Basic auth (Auth ID + Auth Token) and treats 2xx as success (Plivo returns 202 for Message, 201 for Call). The scope is outbound only and the credentials are two secrets.

**Why is this needed**:

Nightingale ships built-in SMS/voice providers such as Tencent and Aliyun, but teams outside those regions have no SMS/voice alert channel and fall back to email or a generic webhook.

Plivo is a global CPaaS to send SMS and place calls, so it fills that gap as an alternative provider without introducing new concepts.

Implementation is drafted and I'm ready to open a PR.
Happy to adjust to your conventions.

Start by reading the existing Tencent and Aliyun providers under alert/sender/provider/ to understand the per-vendor pattern and configuration flow. Then inspect init.go and models/message_tpl.go for provider registration and default templates. Done means adding plivo-sms and plivo-voice with Basic auth, the specified endpoints, and 2xx success handling for SMS and voice delivery.

Written by the indexing model from the issue text.