Add CloudAMQP billing plugin

[KingParmenides]

What does this PR change?

• Adds a CloudAMQP Customer API invoice plugin backed by GET /invoices/period.
• Uses Customer API Basic Auth with an empty username and API key password.
• Downloads each required monthly invoice once per OpenCost request, prorates invoice line totals into daily Custom Cost windows, and falls back to invoice total when line amounts are not provided.
• Adds config validation, README documentation, mocked invoice API tests, and the cloudamqp manifest entry.

Does this PR address any GitHub or Zendesk issues?

• Closes Cloudamqp Plugin #54

How was this PR tested?

• go test ./... from pkg/plugins/cloudamqp
• go vet ./... from pkg/plugins/cloudamqp
• VERSION=dev COMMIT=codex ./tools/build-plugins ./pkg/plugins/cloudamqp/go.mod

Bounty

• This is a submission for the OpenCost plugin bounty.
• BTC address: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ