Add brainpoolP320r1 elliptic curve

[sfarestam-iproov]

Summary

Add brainpoolP320r1 as a named elliptic curve. It is defined in RFC 5639 and fully supported by OpenSSL (NID 929).

Motivation

ICAO Doc 9303 (the international standard for machine-readable travel documents / ePassports), Part 12, Section 4.1.6.3 recommends BSI TR-03111 for elliptic curve selection. TR-03111 includes brainpoolP320r1 as an approved curve for ePassport use.

This curve is deployed in production:

• Netherlands uses brainpoolP320r1 for DG15 Active Authentication keys in ePassport documents

Addressing #4767

I'm aware that a generic request to add Brainpool curves was declined in #4767 as lacking a concrete use case. This request is different: it is driven by a specific international standard (ICAO 9303), backed by real-world data from production ePassport PKI deployments.

Security

brainpoolP320r1 provides 160-bit security, between secp256r1 and secp384r1. ICAO considers it adequate for ePassport documents with 10-year validity.

What this PR does

• Adds BrainpoolP320R1 curve class to the Python API (ec.py)
• Adds OID constant to oid.rs (1.3.36.3.3.2.8.1.1.9)
• Adds NID mappings in the Rust key-parsing and backend modules (ec.rs), gated behind #[cfg(not(any(CRYPTOGRAPHY_IS_BORINGSSL, CRYPTOGRAPHY_IS_AWSLC)))]
• Registers curve in _CURVE_TYPES, _OID_TO_CURVE, and EllipticCurveOID

What this PR does NOT do

• Does not add brainpoolP224r1 (deferred — requires NID addition to the openssl Rust crate first)
• Does not add explicit-parameter-to-named-curve mapping (that is handled separately in Support explicit EC parameters for Brainpool curves #14905)
• Does not change any behavior for BoringSSL or AWS-LC builds

References

• RFC 5639 — Elliptic Curve Cryptography (ECC) Brainpool Standard Curves and Curve Generation
• ICAO Doc 9303 Part 12 — Public Key Infrastructure for MRTDs (Section 4.1.6.3)
• BSI TR-03111 v2.10 — Elliptic Curve Cryptography (recommended by ICAO 9303)
• Dutch CSCA Master List — from the country using brainpoolP320r1
• #4767 — previous curve addition request (rejected for lack of concrete use case)
• #14905 — companion PR adding explicit parameter mapping for existing Brainpool curves