feat(#217): add regionless / region-pair sync for inter-region data transfer

[elecnix]

Summary

Follow-up to #218. Wires up DataTransfer-InterRegion-GB — the one #217 metric left seed-only because it didn't fit the region-scoped Infracost sync.

The problem

AWSDataTransfer products are catalogued globally (region="") with a distinct usagetype per source/destination region pair (e.g. USE1-APS4-AWS-Out-Bytes, transferType="InterRegion Outbound", $0.02/GB). The sync queried region="us-east-1" (→ 0 rows) and stored under the product's region (→ ""), so the metric never resolved.

The mechanism (two optional, additive descriptor keys)

• query_region — query a service's global catalogue (region="") while still storing under the caller's region.
• region_pair_source (+ usagetype_suffix) — collapse the many region-pair rows to one representative rate via _upsert_region_pair_representative:
  1. keep only rows leaving the sync region (usagetype starts with its REGION_PREFIX short code) matching the suffix (-AWS-Out-Bytes),
  2. drop $0 rows (Local Zones / Wavelength / same-metro pairs),
  3. pick the modal price — the region's standard published rate — and store it once, flat, under the sync region.

For us-east-1 this resolves to the standard $0.02/GB inter-region rate, matching the seed. Existing descriptors are untouched (both keys are optional; default behavior unchanged).

Scope note

Only DataTransfer-InterRegion-GB is wired (its grouping was validated against the live API). DataTransfer-Internet-Out-GB and DataTransfer-InterAZ-GB remain seed-only — they use different transferType/usagetype groupings I haven't live-confirmed; they can adopt the same mechanism in a follow-up.

Test plan (HTTP layer mocked)

• query_region: "" actually queries the global catalogue (asserts posted region == "").
• Pair collapse to the modal rate, with source-prefix / suffix / $0 exclusions verified.
• Graceful no-match → 0 upserts.
• Descriptor presence.
• Full suite: 888 passed locally.

Note: the AWSDataTransfer grouping (regionless, transferType="InterRegion Outbound", <PREFIX>-<dst>-AWS-Out-Bytes) was confirmed against the live Cloud Pricing API during development; CI runs fully mocked.