Include the incubating dependency in the published pom as an optional dependency

[trask]

Resolves #7119

The published poms for the related modules, e.g. for io.opentelemetry:opentelemetry-sdk-trace, would now include:

    <dependency>
      <groupId>io.opentelemetry</groupId>
      <artifactId>opentelemetry-api-incubator</artifactId>
      <version>1.48.0-alpha</version>
      <scope>runtime</scope>
      <optional>true</optional>
    </dependency>
  </dependencies>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.85%. Comparing base (56941a5) to head (d1ba7c4).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #7137   +/-   ##
=========================================
  Coverage     89.85%   89.85%           
  Complexity     6610     6610           
=========================================
  Files           740      740           
  Lines         19986    19986           
  Branches       1964     1964           
=========================================
  Hits          17958    17958           
  Misses         1439     1439           
  Partials        589      589           

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