Add campaign CRUD client methods and check management

[jamescarr]

Problem

The OpsLevel GraphQL API exposes full campaign mutation support (campaignCreate, campaignUpdate, campaignDelete, campaignScheduleUpdate, campaignUnschedule, campaignEnd) plus checksCopyToCampaign for associating checks, but the Go SDK only has a ListCampaigns query. There are no client methods for creating, reading, updating, deleting, or scheduling campaigns — which blocks the Terraform provider from managing campaigns as code.

Proposed Solution

Add the following client methods and supporting types:

Client Methods

Method	GraphQL Operation	Description
GetCampaign(id)	account.campaign(id:)	Read a single campaign by ID
CreateCampaign(input)	campaignCreate	Create a draft campaign
UpdateCampaign(input)	campaignUpdate	Update campaign fields
DeleteCampaign(id)	campaignDelete	Delete a campaign
ScheduleCampaign(input)	campaignScheduleUpdate	Set start and target dates
UnscheduleCampaign(id)	campaignUnschedule	Revert to draft
EndCampaign(input)	campaignEnd	End campaign, optionally promote checks
CopyChecksToCampaign(input)	checksCopyToCampaign	Copy rubric checks into a campaign
ListCampaignChecks(id)	account.campaign.checks	List checks belonging to a campaign (paginated)

Input Types

• CampaignCreateInput, CampaignUpdateInput, CampaignScheduleUpdateInput, CampaignEndInput, CampaignUnscheduleInput
• CampaignReminderInput, ChecksCopyToCampaignInput

Additional Types

• CampaignCheckNode (id + name) for the ListCampaignChecks response
• Payload types for each mutation

All methods follow the existing SDK patterns (client.Mutate / client.Query + HandleErrors).

Context

This is needed to support opslevel_campaign as a Terraform resource (see OpsLevel/terraform-provider-opslevel for the companion PR).