chore: allow --api-path as option to generate cmd and generate per api/majorversion

[zhumin8]

This change allow generator to generate for a specified api-path. When "--api-path" is not specified, fallback to "--library-names", and behavior for existing command should not change.

For now, api-path should only take in path to api/version, e.g. google/cloud/functions/v2

-- Manual tests

python library_generation/cli/entry_point.py generate --output=/path/to/empty/output --generation-input=/path/to/gen-input --api-root=/path/to/googleapis --api-path=google/cloud/alloydb/v1

generate output

├── google-cloud-alloydb
│   ├── pom.xml                                                                            
│   ├── src                                                                                
│   │   ├── main                                                                           
│   │   │   ├──......                                                
│   │   │   │    └── v1  
├── google-cloud-alloydb-bom
├── grpc-google-cloud-alloydb-v1
├── owlbot.py
├── pom.xml
├── proto-google-cloud-alloydb-v1
├── README.md
└── samples

[diegomarquezp]

Thanks for the patience!https://github.com/googleapis/sdk-platform-java/pull/3726 has been merged and merging main should fix the generation IT.

[sonarqubecloud[bot]]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud[bot]]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud