Fix declarative configuration example

[jaydeluca]

The declarative config example was broken. I fixed it and added a github action step to run it on PRs so we'll get alerted to this in the future.

This example was failing with:

Task :opentelemetry-examples-declarative-configuration:run FAILED
Exception in thread "main" io.opentelemetry.api.incubator.config.DeclarativeConfigException: Unable to parse configuration input stream
at io.opentelemetry.sdk.extension.incubator.fileconfig.DeclarativeConfiguration.parse(DeclarativeConfiguration.java:142)
at io.opentelemetry.sdk.extension.incubator.fileconfig.DeclarativeConfiguration.parseAndCreate(DeclarativeConfiguration.java:86)
at io.opentelemetry.examples.fileconfig.Application.main(Application.java:31)
Caused by: java.lang.IllegalArgumentException: Cannot construct instance of io.opentelemetry.sdk.extension.incubator.fileconfig.internal.model.TextMapPropagatorModel (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('tracecontext')
at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.opentelemetry.sdk.extension.incubator.fileconfig.internal.model.OpenTelemetryConfigurationModel["propagator"]->io.opentelemetry.sdk.extension.incubator.fileconfig.internal.model.PropagatorModel["composite"]->java.util.ArrayList[0])
at com.fasterxml.jackson.databind.ObjectMapper._convert(ObjectMapper.java:4663)
at com.fasterxml.jackson.databind.ObjectMapper.convertValue(ObjectMapper.java:4594)
at io.opentelemetry.sdk.extension.incubator.fileconfig.DeclarativeConfiguration.parse(DeclarativeConfiguration.java:150)
at io.opentelemetry.sdk.extension.incubator.fileconfig.DeclarativeConfiguration.parse(DeclarativeConfiguration.java:140)
... 2 more
Caused by: com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of io.opentelemetry.sdk.extension.incubator.fileconfig.internal.model.TextMapPropagatorModel (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('tracecontext')
at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.opentelemetry.sdk.extension.incubator.fileconfig.internal.model.OpenTelemetryConfigurationModel["propagator"]->io.opentelemetry.sdk.extension.incubator.fileconfig.internal.model.PropagatorModel["composite"]->java.util.ArrayList[0])...