Generate api.md for Java module

[weidongxu-microsoft]

Combine

• https://github.com/Azure/azure-sdk-tools/tree/main/src/java/apiview-java-processor
• https://github.com/Azure/azure-sdk-tools/blob/main/eng/common/scripts/Export-APIViewMarkdown.ps1 (need small modification)

to generate api.md and put to module root.

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Integration

• integrate into SDK Generation pipeline
• command to generate api.md when user does not use the pipeline (this is what Python does -- another alternative is to have agent on PR to regen the api.md and commit, if not match)

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Question

• How to build the apiview-java-processor, given it currently in the tools repo? Should we move it into SDK repo? Or upload the build to devfeed?
• Later we may modify the processor to directly output markdown, but not a priority right now.

[weidongxu-microsoft]

sample output
https://github.com/Azure/azure-sdk-for-java/pull/50226/changes#diff-5922b3ae4bdbd2ca938e515bb4354db9ac44d1885b732407edc68ced30f5b625