Add Confluent billing plugin

manifest

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 datadog
 mongodb-atlas
 openai
+confluent

pkg/plugins/confluent/README.md

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+# Confluent OpenCost Plugin
+
+The Confluent plugin imports Confluent Cloud billing line items from the `billing/v1/costs` API into OpenCost Custom Costs.
+
+## Configuration
+
+Create a plugin config JSON file with a Confluent Cloud API key and secret:
+
+```json
+{
+  "confluent_api_key": "<cloud-api-key>",
+  "confluent_api_secret": "<cloud-api-secret>",
+  "log_level": "info"
+}
+```
+
+Optional fields:
+
+```json
+{
+  "api_base_url": "https://api.confluent.cloud",
+  "page_size": 5000
+}
+```
+
+`page_size` defaults to `5000`, matching the Confluent costs API default, and may be set up to `10000`.
+
+## Cost Mapping
+
+The plugin queries `/billing/v1/costs` once per daily OpenCost window with `start_date` and `end_date`, follows `metadata.next` pagination links, and maps each Confluent billing line item into a Custom Cost:
+
+- `amount` -> billed cost
+- `original_amount` -> list cost
+- `quantity` and `unit` -> usage quantity and unit
+- `product` -> service and resource type
+- `line_type` -> SKU ID
+- resource environment ID -> account name and account ID
+- resource ID/display name -> sub-account ID/name
+
+Only daily resolution is supported because Confluent returns costs at daily granularity.