feat(loadtesting): add Load Testing capability index (regression)

Adds capability/loadtesting.capability-index.json in the per-product format
(#387): product at top level, version field, and
schemas/responses/paging tables with per-capability response refs. 19
capabilities over load-api /api/v1/agent/* and 9 entities. base_url baked to
the regression host (load-testing-rengg-lts.bsstag.com) for this branch.

Author: sourabhd-cbu