Add helpers for creating channel args.

src/core/ext/filters/client_channel/client_channel_factory.c

src/core/ext/filters/client_channel/client_channel_plugin.c

src/core/ext/filters/client_channel/http_proxy.c

...core/ext/filters/client_channel/lb_policy/grpclb/grpclb.c

src/core/ext/filters/client_channel/lb_policy_factory.c

src/core/ext/filters/client_channel/subchannel.c

src/core/ext/filters/load_reporting/load_reporting.c

...ore/ext/transport/chttp2/client/insecure/channel_create.c

...t/transport/chttp2/client/insecure/channel_create_posix.c

...xt/transport/chttp2/client/secure/secure_channel_create.c

src/core/lib/channel/channel_args.c

src/core/lib/channel/channel_args.h

src/core/lib/http/httpcli.c

src/core/lib/iomgr/socket_factory_posix.c

src/core/lib/iomgr/socket_mutator.c

src/core/lib/security/context/security_context.c

src/core/lib/security/credentials/credentials.c

src/core/lib/security/credentials/fake/fake_credentials.c

src/core/lib/security/credentials/ssl/ssl_credentials.c

src/core/lib/security/transport/lb_targets_info.c