Visualization of sequential stage groups using declarative pipelines need BlueOcean nodes API response DAG to include the sequential stages.

• See syntax with example: https://github.com/jenkins-infra/jenkins.io/pull/1274/files#diff-fa72dbc25a0d4d142e78fc74f56be75dR1070 in PR https://github.com/jenkins-infra/jenkins.io/pull/1274/
• Nodes API is implemented in PipelineNodeGraphVisitor. First thing should be to check FlowNode representation of sequential and group stages.
• Assuming stages == parallel-branch is true, existing implementation ignores nested stages inside parallels, so some work there to make sure we play with nested scripted pipeline and declarative as well - maybe somehow detect at runtime if its declarative and then handle it differently.