SDK: Expose C bindings

[inge4pres]

Reason for this PR

Create C exports, statically linked library and relevant header file to allow C programs to consume the Zig implementation of the SDK.

Closes #56.

Details

Exposing Zig code to C was not as easy as I thought 😅
There are several limitations that I encountered and I had not foreseen simply due to my lack of experience on the matter (Dunning-Krueger?).

Nevertheless, the PR:

• Mirrors the SDK Zig types to export new types in c.zig
• Re-defines enums where needed
• Forces the use f std.heap.c_allocator on the C exported code
• Adds examples, compiling and running as part of a new step, and using them as validation tests (zig buildexample-c)

Additional context

We should be more explicit that the C API is not going to be stable even when we stabilize the SDK.
The reason is: depending on the type of modifications we apply to the Zig SDK code, we might not reflect them immediately to the C code (it might require additional changes in the C part).
We cannot require contributors that changes to the Zig code are updated on the C part in the same PR, hence we are not going to flag the C API as stable.

[inge4pres]

Proud to have accomplished this

☁  opentelemetry-sdk [c-interop] nm -g zig-out/lib/libopentelemetry-sdk.a 2>&1 | grep otel_
00000000000081d0 T otel_counter_add_u64
000000000000ca00 T otel_gauge_record_f64
000000000000b1e0 T otel_histogram_record_f64
000000000029ce10 T otel_logger_emit
000000000029d4f0 T otel_logger_is_enabled
000000000029bc90 T otel_logger_provider_add_log_record_processor
000000000029a3d0 T otel_logger_provider_create
000000000029c450 T otel_logger_provider_force_flush
000000000029b520 T otel_logger_provider_get_logger
000000000029a9b0 T otel_logger_provider_shutdown
000000000029d880 T otel_log_record_exporter_stdout_create
0000000000006ef0 T otel_meter_create_counter_u64
000000000000bed0 T otel_meter_create_gauge_f64
000000000000a6b0 T otel_meter_create_histogram_f64
0000000000008eb0 T otel_meter_create_updown_counter_i64
0000000000004c10 T otel_meter_provider_add_reader
0000000000003be0 T otel_meter_provider_create
0000000000004490 T otel_meter_provider_get_meter
0000000000003d00 T otel_meter_provider_init
0000000000003e50 T otel_meter_provider_shutdown
000000000000e310 T otel_metric_exporter_inmemory_create
000000000000da40 T otel_metric_exporter_stdout_create
000000000004b8c0 T otel_metric_reader_collect
000000000000ee20 T otel_metric_reader_create
000000000004bb00 T otel_metric_reader_shutdown
000000000029db50 T otel_simple_log_record_processor_create
0000000000251dc0 T otel_simple_span_processor_create
000000000024ee80 T otel_span_add_event
000000000024f5b0 T otel_span_add_event_with_timestamp
000000000024d380 T otel_span_end
000000000024d500 T otel_span_end_with_timestamp
0000000000251910 T otel_span_exporter_stdout_create
0000000000251650 T otel_span_get_span_id_hex
0000000000251340 T otel_span_get_trace_id_hex
00000000002511c0 T otel_span_is_recording
0000000000250dd0 T otel_span_record_exception
000000000024e300 T otel_span_set_attribute_bool
000000000024dfa0 T otel_span_set_attribute_double
000000000024dc50 T otel_span_set_attribute_int
000000000024d930 T otel_span_set_attribute_string
000000000024fd50 T otel_span_set_status
0000000000250080 T otel_span_update_name
000000000024d260 T otel_tracer_is_enabled
000000000024af90 T otel_tracer_provider_add_span_processor
00000000002499e0 T otel_tracer_provider_create
000000000024b730 T otel_tracer_provider_force_flush
000000000024a830 T otel_tracer_provider_get_tracer
0000000000249f80 T otel_tracer_provider_shutdown
000000000024c270 T otel_tracer_start_span
00000000000099d0 T otel_updown_counter_add_i64