Enhancement: Allow MergedSpecBuilder title, version and description to be configured to fix [#20822]

[saulgillEST]

Allow MergedSpecBuilder title, version and description to be configured to fix #20822

I have made small changes to MErgedSpecBuilder and CodeGenMojo to allow users of the openapi-generator-maven-plugin to add a custom info header in merged open api files.

Before making this change, the fields in the info header were hard-coded as title="merged spec", description="merged spec" and version=1.0.0

Now, through the use of 3 maven plugin tags, the setting of these fields by the user of the plugin is now possible. An example is provided below. With these changes, one can easily verify that the new plugin configuration is being adopted. The new info header values will appear in the open api file and any generated documentation too.

<configuration>
	<inputSpecRootDirectory>some-directory</inputSpecRootDirectory>
	<mergedFileName>openapi-file</mergedFileName>  <!-- Merged output file -->
	<mergedFileInfoName>My Info Name/Title</mergedFileInfoName>
	<mergedFileInfoDescription>My Description</mergedFileInfoDescription>
	<mergedFileInfoVersion>1.0.0</mergedFileInfoVersion>
	<generatorName>openapi-yaml</generatorName>  <!-- Change based on required language -->
</configuration>`

The default values remain the same as before. Old functionality is not impacted at all.

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
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[JohnKeeneyEST]

Looks good. Thanks.