Get splitTestAssignment from SDK bundle, if provided

[jplaroche]

and doesn't override if splitTestAssignment is already set (in case it is set server-side)

Why

Split-test assignment tagging (ortb2Imp.ext.optable.splitTestAssignment) is currently copy-pasted as a trackAuctionEnd monkey-patch across at least 4 SDK bundles (ezoic, britannica, publift, aditude — the latter merged just this week, #612). Each bundle wraps analytics(_v2).trackAuctionEnd identically to inject the same field before analytics processes the auction. Centralizing this in the SDK removes the duplication and the drift risk of bundles copy-pasting it slightly differently.

What Changed

• addons/prebid/analytics.ts (v2) / addons/prototypes/analytics.js (v1): added optional getSplitTestAssignment?: () => string | undefined config callback.
• trackAuctionEnd: when configured, tags every bid's ortb2Imp.ext.optable.splitTestAssignment with the callback's return value — skipped if the bid already has a splitTestAssignment set (e.g. set server-side upstream), so this never clobbers an existing value.
• No-op (existing behavior unchanged) when getSplitTestAssignment is omitted.
• Mapping/semantics of the assignment value (e.g. a bundle's 'none' → 'test' relabeling) intentionally stay owned by the calling bundle, not baked into the SDK — the callback returns the final string to tag.

How to Test

• npx jest lib/addons/prebid/analytics.test.ts — 5 new cases under trackAuctionEnd - split test assignment (no-op when unconfigured, tags all bids, does not override existing assignment, preserves other ext.optable fields). Full suite: 452/452 passing.
• Tested
• Docs / README updated (if public API changed)

Notes

• Not breaking — new optional config field, fully backward compatible when omitted.
• Follow-up (separate PRs, per bundle, after this ships): migrate ezoic, britannica, publift, aditude off their local trackAuctionEnd monkey-patch onto this config option.
• Breaking change
• Requires release - new SDK version needed before bundles can consume this