feat(integrations): add Plain API support (#3028)

## Describe your changes
Adding configuration for Plain API Key

## Issue ticket number and link

## Checklist before requesting a review (skip if just adding/editing
APIs & templates)
- [ ] I added tests, otherwise the reason is:  no sync/actions yet
- [ ] I added observability, otherwise the reason is: no sync/actions
yet
- [ ] I added analytics, otherwise the reason is:  no sync/actions yet

docs-v2/integrations/all/plain.mdx

@@ -0,0 +1,31 @@
+---
+title: Plain
+sidebarTitle: Plain
+---
+
+API configuration: [`plain`](https://nango.dev/providers.yaml)
+
+## Features
+
+| Features | Status |
+| - | - |
+| [Auth (API Key)](/integrate/guides/authorize-an-api) | ✅ |
+| [Sync data](/integrate/guides/sync-data-from-an-api) | 🚫 |
+| [Perform workflows](/integrate/guides/perform-workflows-with-an-api) | 🚫 |
+| [Proxy requests](/integrate/guides/proxy-requests-to-an-api) | 🚫 |
+| [Receive webhooks](/integrate/guides/receive-webhooks-from-an-api) | 🚫 |
+
+<Tip>We can implement missing features in &lt;48h, just ask for it in the [community](https://nango.dev/slack).</Tip>
+
+## Getting started
+
+-   [Plain API docs](https://www.plain.com/docs/quickstart)
+-   [Authentication and API key](https://www.plain.com/docs/api-reference/graphql/authentication)
+
+<Tip>Need help getting started? Get help in the [community](https://nango.dev/slack).</Tip>
+
+## API gotchas
+
+- Plain uses API_KEY auth mode with `Authorization: Bearer '{API_KEY}'` in the request header to access different endpoints.
+
+<Note>Add Getting Started links and Gotchas by [editing this page](https://github.com/nangohq/nango/tree/master/docs-v2/integrations/all/ragieai.mdx)</Note>

docs-v2/integrations/support.mdx

@@ -25,6 +25,7 @@ sidebarTitle: Support
     <Card title="HubSpot" href="/integrations/all/hubspot" color="#68a063" />
     <Card title="Intercom" href="/integrations/all/intercom" color="#68a063" />
     <Card title="Medallia" href="/integrations/all/medallia" color="#68a063" />
+    <Card title="Plain" href="/integrations/all/plain" color="#68a063" />
     <Card title="RingCentral" href="/integrations/all/ring-central" color="#68a063" />
     <Card title="StackExchange" href="/integrations/all/stackexchange" color="#68a063" />
     <Card title="Zendesk" href="/integrations/all/zendesk" color="#68a063" />

docs-v2/mint.json

@@ -516,6 +516,7 @@
                 "integrations/all/pinterest",
                 "integrations/all/pipedrive",
                 "integrations/all/pivotaltracker",
+                "integrations/all/plain",
                 "integrations/all/productboard",
                 "integrations/all/podium",
                 "integrations/all/posthog",

packages/shared/providers.yaml

@@ -4641,6 +4641,22 @@ pivotaltracker:
             title: API Key
             description: The API key for your Pivotal Tracker account
 
+plain:
+    display_name: Plain
+    categories:
+        - support
+    auth_mode: API_KEY
+    proxy:
+        headers:
+            Authorization: Bearer ${apiKey}
+        base_url: https://core-api.uk.plain.com/graphql/v1
+    docs: https://docs.nango.dev/integrations/all/plain
+    credentials:
+        apiKey:
+            type: string
+            title: API Key
+            description: The API key for your Plain account
+
 podium:
     display_name: Podium
     categories: