Support explicit EC parameters for Brainpool curves

[sfarestam-iproov]

Summary

Extend the explicit-to-named-curve mapping (added in #12916) to cover brainpoolP256r1, brainpoolP384r1, and brainpoolP512r1. These curves are already supported as named curves — this PR adds only the SpecifiedECDomain constants so that keys encoded with explicit domain parameters are recognized and mapped to the existing named curves.

Motivation

ICAO Doc 9303 (the international standard for machine-readable travel documents / ePassports), Part 12, Section 4.1.6.3 states:

"The elliptic curve domain parameters used to generate the ECDSA key pair MUST be described explicitly in the parameters of the public key, i.e. parameters MUST be of type ECParameters (no named curves, no implicit parameters) and MUST include the optional co-factor. ECPoints MUST be in uncompressed format."

The standard further recommends BSI TR-03111 for elliptic curve selection, which includes the Brainpool curves defined in RFC 5639.

Countries including Germany, Sweden, Finland, Switzerland, Austria, Cyprus, Lithuania, Latvia, Brazil, Russia, China, Turkey, and 20+ others issue ePassport CSCA and Document Signer certificates using Brainpool curves with explicit parameters, exactly as the standard mandates. These certificates are available in the publicly downloadable German CSCA Master List (580 certificates from 114 countries) and the Dutch CSCA Master List.

What this PR does

• Adds BRAINPOOLP256R1_DOMAIN, BRAINPOOLP384R1_DOMAIN, BRAINPOOLP512R1_DOMAIN constants (and _NO_SEED variants) to ec_constants.rs, with domain parameters from RFC 5639 Section 3
• Adds match arms in the SpecifiedCurve branch of ec_params_to_group in ec.rs, gated behind #[cfg(not(any(CRYPTOGRAPHY_IS_BORINGSSL, CRYPTOGRAPHY_IS_AWSLC)))] (same as the existing named-curve Brainpool entries)
• Adds test vectors generated with openssl ecparam -name <curve> -param_enc explicit [-no_seed] -genkey
• Extends the existing test_load_private_key_explicit_parameters and test_load_public_key_explicit_parameters parametrized tests

What this PR does NOT do

• Does not add any new curves — brainpoolP256r1, brainpoolP384r1, brainpoolP512r1 are already in the library
• Does not support arbitrary/custom explicit curves — only exact byte-level matches against RFC 5639 constants
• Does not change any behavior for BoringSSL or AWS-LC builds

References

• ICAO Doc 9303 Part 12 — Public Key Infrastructure for MRTDs (Section 4.1.6.3, p.16)
• RFC 5639 — Elliptic Curve Cryptography (ECC) Brainpool Standard Curves and Curve Generation
• BSI TR-03111 v2.10 — Elliptic Curve Cryptography (recommended by ICAO 9303)
• German CSCA Master List — 580 certificates from 114 countries (test data source)
• Dutch CSCA Master List — additional test data source
• #5659 — original issue requesting explicit EC parameter support (filed for ICAO 9303)
• #12916 — PR that added explicit parameter mapping for P-256/P-384/P-521