### Description of changes:
gRPC recently landed a TLS Private Key Offload feature (grpc/grpc#41606)
that gates BoringSSL-specific code paths behind `#ifdef
OPENSSL_IS_BORINGSSL`. AWS-LC defines `OPENSSL_IS_AWSLC` instead, so
gRPC doesn't recognize it as BoringSSL-compatible. This causes all 11
`TlsPrivateKeyOffloadTest` tests to fail at setup time because
`grpc_tls_identity_pairs_add_pair_with_signer` returns
`UnimplementedError`.

Since AWS-LC is a BoringSSL fork and supports all the relevant APIs
(e.g. `SSL_PRIVATE_KEY_METHOD`), we pass `-DOPENSSL_IS_BORINGSSL=1` via
`CMAKE_C_FLAGS`/`CMAKE_CXX_FLAGS` when building gRPC. This enables the
BoringSSL code paths that are correct for AWS-LC, including private key
offload, optimized session caching, and type macros.

### Call-outs:
All `OPENSSL_IS_BORINGSSL` checks in gRPC are preprocessor-level guards
inside source files — there's no CMake-level source file selection based
on BoringSSL detection, so the CFLAGS approach is safe.

### Testing:
The gRPC integration test itself validates the fix — the 11
`TlsPrivateKeyOffloadTest` tests that were failing should now pass.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

### Description of changes:
gRPC recently landed a TLS Private Key Offload feature (grpc/grpc#41606)
that gates BoringSSL-specific code paths behind `#ifdef
OPENSSL_IS_BORINGSSL`. AWS-LC defines `OPENSSL_IS_AWSLC` instead, so
gRPC doesn't recognize it as BoringSSL-compatible. This causes all 11
`TlsPrivateKeyOffloadTest` tests to fail at setup time because
`grpc_tls_identity_pairs_add_pair_with_signer` returns
`UnimplementedError`.

Since AWS-LC is a BoringSSL fork and supports all the relevant APIs
(e.g. `SSL_PRIVATE_KEY_METHOD`), we pass `-DOPENSSL_IS_BORINGSSL=1` via
`CMAKE_C_FLAGS`/`CMAKE_CXX_FLAGS` when building gRPC. This enables the
BoringSSL code paths that are correct for AWS-LC, including private key
offload, optimized session caching, and type macros.

### Call-outs:
All `OPENSSL_IS_BORINGSSL` checks in gRPC are preprocessor-level guards
inside source files — there's no CMake-level source file selection based
on BoringSSL detection, so the CFLAGS approach is safe.

### Testing:
The gRPC integration test itself validates the fix — the 11
`TlsPrivateKeyOffloadTest` tests that were failing should now pass.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

Closes grpc#41606

COPYBARA_INTEGRATE_REVIEW=grpc#41606 from anniefrchz:private_key_signer 881d900
PiperOrigin-RevId: 876385877

### Description of changes:
gRPC recently landed a TLS Private Key Offload feature (grpc/grpc#41606)
that gates BoringSSL-specific code paths behind `#ifdef
OPENSSL_IS_BORINGSSL`. AWS-LC defines `OPENSSL_IS_AWSLC` instead, so
gRPC doesn't recognize it as BoringSSL-compatible. This causes all 11
`TlsPrivateKeyOffloadTest` tests to fail at setup time because
`grpc_tls_identity_pairs_add_pair_with_signer` returns
`UnimplementedError`.

Since AWS-LC is a BoringSSL fork and supports all the relevant APIs
(e.g. `SSL_PRIVATE_KEY_METHOD`), we pass `-DOPENSSL_IS_BORINGSSL=1` via
`CMAKE_C_FLAGS`/`CMAKE_CXX_FLAGS` when building gRPC. This enables the
BoringSSL code paths that are correct for AWS-LC, including private key
offload, optimized session caching, and type macros.

### Call-outs:
All `OPENSSL_IS_BORINGSSL` checks in gRPC are preprocessor-level guards
inside source files — there's no CMake-level source file selection based
on BoringSSL detection, so the CFLAGS approach is safe.

### Testing:
The gRPC integration test itself validates the fix — the 11
`TlsPrivateKeyOffloadTest` tests that were failing should now pass.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

Closes grpc#41606

COPYBARA_INTEGRATE_REVIEW=grpc#41606 from anniefrchz:private_key_signer 881d900
PiperOrigin-RevId: 876385877