add campaign api (#274)

Author: KMKoushik

apps/docs/api-reference/campaigns/create-campaign.mdx

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+---
+openapi: post /v1/campaigns
+---

apps/docs/api-reference/campaigns/get-campaign.mdx

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+---
+openapi: get /v1/campaigns/{campaignId}
+---

apps/docs/api-reference/campaigns/pause-campaign.mdx

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+---
+openapi: post /v1/campaigns/{campaignId}/pause
+---

apps/docs/api-reference/campaigns/resume-campaign.mdx

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+---
+openapi: post /v1/campaigns/{campaignId}/resume
+---

apps/docs/api-reference/campaigns/schedule-campaign.mdx

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+---
+openapi: post /v1/campaigns/{campaignId}/schedule
+---