Support Creators API credential v3.x

[Oakes6]

Summary

Extends the client so v3.1 / v3.2 / v3.3 credentials work end-to-end: correct Login with Amazon token URLs, HTTP Basic client authentication, scope creatorsapi::default, and catalog Authorization: Bearer <token> without a Version suffix. v2.1 / v2.2 / v2.3 (Cognito) behavior is unchanged and remains available via WithCredentialVersion and the existing constants.

Motivation

New Associates Central credential sets use the v3.x path. The library previously only mapped 2.x to Cognito; 3.x strings were rejected by WithCredentialVersion, and defaults assumed Cognito, so v3 credentials failed token exchange or API auth.

What changed

• marketplace.go: Added CredentialVersionNAv3 (3.1), CredentialVersionEUv3 (3.2), CredentialVersionFEv3 (3.3). versionMap now defaults each built-in locale to the matching v3 code (same NA/EU/FE grouping as before).
• server.go: authEndpointMap entries for LwA (api.amazon.com, api.amazon.co.uk, api.amazon.co.jp), isLWACredentialVersion, and CreateClient wires lwaFlow into the token manager.
• auth.go: tokenManager branches: Cognito (form client_id / client_secret, scope creatorsapi/default) vs LwA (Basic auth, scope creatorsapi::default). authorizationHeader drops , Version … for LwA.
• client.go: Stores lwaFlow and passes it into authorizationHeader for catalog requests.
• Tests: Default client path asserts LwA token request shape and bare Bearer catalog header; TestClientLegacyCognitoCredentialVersion pins 2.x Cognito + Version behavior.
• README / examples: Document v2 vs v3, default 3.1 / 3.3 in snippets, and legacy override via WithCredentialVersion("2.1"|…).

Migration / breaking note

Default credential version per marketplace is now v3.x, matching typical new credentials. Integrations still on Cognito-only 2.x credentials must set e.g. WithCredentialVersion(paapi5.CredentialVersionNA) (or "2.2" / "2.3") on CreateClient.

How to verify

go test ./...
golangci-lint run

[Oakes6]

@spiegel-im-spiegel Adding default support for v3 generated credentials. This defaults to v3 with options for auth'ing with v2. Amazon changing things at a break-neck pace right now.

[spiegel-im-spiegel]

Thanks for the update and for moving quickly on the v3 credential support work. Merged now — really appreciate the contribution.