Package-level declarations

Per-country address spec — what the merchant's address form should look like for each ISO country code.

E.164: leading + or start immediately, country digit 1-9, then 1-14 more digits (15 max total). Whitespace and formatting characters are NOT allowed — callers should strip them first if needed.

Verify a Billing payload satisfies every field in requiredFields. Returns null when valid, or an InvalidParamsException describing the first missing/ invalid field. Mirrors the iOS validation rules in AWXDefaultProvider+Extensions.swift.