feat(orders): add order resource

[toommz]

Propagates the Orders API (lago-openapi#lago-1700) to the Ruby client.

Stacked on #369 — based on lago-1699, so the diff here is orders only. GitHub will retarget it as the stack merges.

Adds client.orders — get, get_all, execute(id, params).

execute takes an optional { order: { execution_mode } } body, the same wrapped shape as mark_as_signed in #369. It runs synchronously and is idempotent, so the response carries the resulting status and, on failure, the reasons in execution_record.errors.

bundle exec rspec (577 examples) and bundle exec rubocop are green.

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