Add Global Profile API support

[tholovina-sift]

Summary

Adds client support for the new Global Profile API, which surfaces cross-tenant
identity, decision, chargeback, order, transaction, and location signals for a
user.

Two new endpoints are wired up under the existing account-scoped /v3/accounts
API surface, following the same conventions as get_user_decisions,
get_a_psp_merchant_profile, etc. (HTTP Basic auth via api_client, a
Sift::Response wrapper, account_id/api_key/timeout overrides via opts).

Changes

• lib/sift.rb: Added global_profile_api_path(account_id, user_id) and
  global_profile_lookup_api_path(account_id) path helpers.
• lib/sift/client.rb:
  • get_global_profile(user_id, opts = {}) — GET
    /v3/accounts/{account_id}/global_profile/users/{user_id}. Supports
    :global_only and :include_own_data optional query params. Raises if
    user_id, api_key, or account_id is empty.
  • get_global_profile_by_attributes(params = {}, opts = {}) — POST
    /v3/accounts/{account_id}/global_profile/lookup with a JSON body of
    {email, phone}. Raises "email or phone must be provided" if neither
    :email nor :phone is present in params, mirroring existing
    client-side validation patterns (e.g. properties cannot be empty).
• spec/unit/client_global_profile_spec.rb (new): webmock-based specs for
  both endpoints — happy path, optional query params, identity_found: false
  response shape (all fields null except profile_summary), and the two
  validation error cases.
• README.md: New "Global Profile API" usage section, in the same style
  as the existing PSP Merchant Management API section.
• HISTORY: New 4.7.0 entry — "Global Profile API support".
• lib/sift/version.rb: Bumped VERSION to 4.7.0.

Testing

Unit specs added in spec/unit/client_global_profile_spec.rb. All new/modified
Ruby files pass ruby -c syntax checks. I was unable to run the full RSpec
suite in this environment (no network access to install missing gems via
bundle install — rspec, webmock, httparty, etc. were not preinstalled),
so please run bundle exec rspec in CI/locally to confirm before merging.

Not included

• No changes to test_integration_app or examples/ — happy to add an example
  if desired.