Add `subject` and `runtime_context` in tasks and workflows

For each task and workflows: add subject and runtime_context in the dynamic data.

See: https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/task-statistics.html#collection-debusine-task-history

Worker tasks. Subject and runtime context:

• AssembleSignedSource
• Autopkgtest
• Blhc
• DebDiff
• ExtractForSigning
• Lintian
• MakeSourcePackageUpload
• MergeUploads
• MmDebstrap
• Noop
• Piuparts
• Sbuild
• SimpleSystemImageBuild
• SystemBootstrap
• SystemImageBuild

(15 tasks)

Tasks with configuration context:

• Autopkgtest
• Lintian
• MmDebstrap
• Piuparts
• Sbuild
• SimpleSystemImageBuild
• SystemBootstrap
• SystemImageBuild

(8 tasks)

See #679 (comment 573758) for suggestion of subject/runtime_context/configuration_context.

Workflows:

@hertzog in #679 (comment 573692):

For workflows, we probably don't care about runtime_context. We don't really store runtime statistics about workflows. However it does make sense to be able to override configuration for workflows.

• AutopkgtestWorkflow
• DebianPipelineWorkflow
• LintianWorkflow
• MakeSignedSourceWorkflow
• NoopWorkflow
• PackagePublishWorkflow
• PackageUploadWorkflow
• PiupartsWorkflow
• QAWorkflow
• ReverseDependenciesAutopkgtestWorkflow
• SbuildWorkflow
• UpdateEnvironmentsWorkflow

(12 workflows)

For server/signing/wait tasks: set subject and run_context set to None for now (#679 (comment 573692)).

Part of: #510 (closed)