feat(common): add schema_url to resource.

[TommyCpp]

Part of #711.

Note that when merging two resources:

If the old resource's Schema URL is empty then the resulting resource's Schema URL will be set to the Schema URL of the updating resource,
Else if the updating resource's Schema URL is empty then the resulting resource's Schema URL will be set to the Schema URL of the old resource,
Else if the Schema URLs of the old and updating resources are the same then that will be the Schema URL of the resulting resource,
Else this is a merging error (this is the case when the Schema URL of the old and updating resources are not empty and are different). The resulting resource is undefined, and its contents are implementation-specific.

For the last cases, merging error. The current implementation set the schema_url of the result to None.

Block on #776

[codecov]

Codecov Report

Merging #775 (79dca72) into main (a474268) will increase coverage by 0.1%.
The diff coverage is 93.3%.

@@           Coverage Diff           @@
##            main    #775     +/-   ##
=======================================
+ Coverage   69.8%   69.9%   +0.1%     
=======================================
  Files        109     109             
  Lines       8912    8968     +56     
=======================================
+ Hits        6226    6277     +51     
- Misses      2686    2691      +5     

Impacted Files	Coverage Δ
opentelemetry-sdk/src/resource/mod.rs	92.5% <93.3%> (+<0.1%)	⬆️
opentelemetry-sdk/src/metrics/controllers/push.rs	80.0% <0.0%> (-3.4%)	⬇️
opentelemetry-datadog/src/exporter/mod.rs	10.6% <0.0%> (-0.1%)	⬇️
opentelemetry-otlp/src/span.rs	0.0% <0.0%> (ø)
opentelemetry-sdk/src/trace/span.rs	93.4% <0.0%> (ø)
opentelemetry-sdk/src/trace/tracer.rs	90.7% <0.0%> (+<0.1%)	⬆️
opentelemetry-zipkin/src/exporter/model/span.rs	94.0% <0.0%> (+0.1%)	⬆️
opentelemetry-dynatrace/src/transform/metrics.rs	83.6% <0.0%> (+0.1%)	⬆️
opentelemetry-datadog/src/exporter/model/mod.rs	74.5% <0.0%> (+0.4%)	⬆️
opentelemetry-sdk/src/testing/trace.rs	67.5% <0.0%> (+0.4%)	⬆️

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[jtescher]

Would it make sense to have this be Option<String>? or maybe more optimally Option<&str>? schema_url being a valid URL or None may be less error prone than having it be a valid URL or an empty string.

[TommyCpp]

Sounds reasonable 👍 Updated

[jtescher]

Cool, could we get &str here? or does that cause issues