Add Databricks billing plugin

[KingParmenides]

What does this PR change?

• Adds a Databricks account billable usage plugin backed by GET /api/2.0/accounts/{account_id}/usage/download.
• Downloads each required month once per OpenCost request, parses Databricks CSV usage logs, filters records into daily Custom Cost windows, and maps account/workspace/SKU/product/resource fields into Custom Costs.
• Adds configurable SKU unit pricing because Databricks actual costs vary by customer contract.
• Adds config validation, README documentation, mocked CSV/API tests, and the databricks manifest entry.

Does this PR address any GitHub or Zendesk issues?

• Closes Databricks Plugin #46

How was this PR tested?

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

Bounty

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