Add google_apigee_custom_report resource

[ogormans-deptstack]

New resource google_apigee_custom_report for managing Apigee Analytics custom report definitions via the organizations.reports API.

Custom reports let users define reusable analytics views with custom metrics, dimensions, filters, and grouping. This resource supports all fields from the API including metrics with aggregation functions, dimensions, time units, sorting, and report properties.

Tested all CRUD operations against a live Apigee evaluation org:

• Create (1s)
• Read with no drift
• Update in-place (display name, metrics, dimensions, time unit)
• Import by full resource path
• Delete

Uses PUT with no update mask (same pattern as DeveloperApp). The name field is a server-assigned UUID used as the path identifier. displayName is user-specified and mutable.

Related to PR #16954 (google_apigee_data_collector).

Release Note Template for Downstream PRs (will be copied)

apigee: added `google_apigee_custom_report` resource

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

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