fix(auth): honor funder address in CLOB auth headers

[SebastianBoehler]

Summary

• use the supplied funder/proxy address when building L1 CLOB auth headers for API-key create/derive
• use the same supplied funder/proxy address in L2 HMAC headers when provided
• add V2 signer tests covering funder-aware L1 and L2 header addresses
• document the auth behavior in the V2 migration notes

Context

Konstantin reported that TypeScript API-key derivation worked when funderAddress was set to the portfolio/proxy address, while this C++ client could not derive credentials. The C++ path accepted a funder argument but ignored it in auth header generation.

Validation

• cmake -S . -B /tmp/polymarket-cpp-client-auth-verify -DPOLYMARKET_CLIENT_BUILD_EXAMPLES=ON -DPOLYMARKET_CLIENT_BUILD_TESTS=ON -DPOLYMARKET_CLIENT_BUILD_BENCHMARKS=ON
• cmake --build /tmp/polymarket-cpp-client-auth-verify -- -j4
• ctest --test-dir /tmp/polymarket-cpp-client-auth-verify --output-on-failure
• git diff --check