Updated trace_context.proto and associated files.

src/core/ext/census/gen/trace_context.pb.c

src/core/ext/census/gen/trace_context.pb.h

src/core/ext/census/trace_context.c

src/core/ext/census/trace_context.h

src/proto/census/trace_context.proto

test/core/census/data/context_full.pb

test/core/census/data/context_full.txt

test/core/census/data/context_no_sample.pb

test/core/census/data/context_no_sample.txt

test/core/census/data/context_no_span_options.pb

test/core/census/data/context_no_span_options.txt

test/core/census/data/context_span_only.pb

test/core/census/data/context_span_only.txt

test/core/census/data/context_trace_only.pb

test/core/census/data/context_trace_only.txt

test/core/census/trace_context_test.c