Add BRCode resource

[lucasmiranda-stark]

Summary

• Adds StarkInfra.BrcodePreview — single batch create/2 operation against POST /brcode-preview, returning previewed BR Code payloads with all 23 server-populated fields.
• Adds embedded StarkInfra.BrcodePreview.Subscription sub-resource (18 output-only fields) parsed into the parent's :subscription field when the previewed BR Code carries Pix-recurring-debit metadata.
• Patches StarkInfra.Utils.Check.datetime/1, date/1, and date_or_datetime/1 to normalize empty strings ("") returned by the server to nil, matching the parent contract's M5 normalization (also unblocks future resources that hit the same edge case).
• Adds :ssl and :public_key to extra_applications in mix.exs so the test suite can boot SSL/TLS on Erlang/OTP 28 (:httpc.ssl_verify_host_options/1 eagerly calls :public_key before the runtime ensure_all_started(:ssl) finishes). No production behaviour change — client apps already start :ssl from their own supervision tree.
• Adds test/utils/static_brcode.exs test helper that mints a fresh sandbox StaticBrcode via Request.fetch(:post, "/static-brcode", ...), used by the new test/brcode_preview_test.exs so the e2e tests self-bootstrap their own input BR Code (mirrors the Python SDK pattern).

Test plan

• mix test --only brcode_preview — 2 tests, 0 failures on the Infra sandbox.
• mix test --only pix_balance — smoke test still green (no regression from the mix.exs change).
• Reviewer: confirm the Check.datetime("") head is the right place for the empty-string normalization (vs. inline at each call-site).
• Reviewer: confirm extra_applications: [:inets, :ssl, :public_key] is acceptable, or whether you'd prefer a mod: {StarkInfra.Application, []} start-up hook.

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