Move string.h to internal code

src/core/channel/call_op_string.c

src/core/channel/channel_args.c

src/core/channel/client_channel.c

src/core/channel/connected_channel.c

src/core/httpcli/httpcli.c

src/core/httpcli/httpcli_security_context.c

src/core/iomgr/resolve_address_posix.c

src/core/iomgr/sockaddr_utils.c

src/core/iomgr/socket_utils_common_posix.c

src/core/iomgr/tcp_posix.c

src/core/security/credentials.c

src/core/security/json_token.c

src/core/security/secure_endpoint.c

src/core/security/security_context.c

src/core/statistics/census_rpc_stats.c

src/core/statistics/census_tracing.c

src/core/support/cmdline.c

src/core/support/host_port.c

src/core/support/string.c

src/core/support/string.h