feat(decl): add explain command

[ekoops]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind documentation

/kind tests

/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area commands

/area pkg

/area events

What this PR does / why we need it:

This PR adds the explain command to the declarative namespace. The explain command can produce documentation for different YAML tests description properties, as requested by the user. The following are some invocation examples:

# Generate documentation for properties directly available under the YAML document root
event-generator declarative explain

# Generate documentation for all properties in the property path leading to the `processes`
# property (i.e.: document `tests`, `context` and `processes` properties), as well as any property
# available under `processes`
event-generator declarative explain tests.context.processes

# Generate documentation for all properties in the property path leading to the `how` property
# (i.e.: document `tests`, `steps`, `args` and `how` properties), as well as any property available
# under `how`.
event-generator declarative explain tests.steps{type=syscall}{syscall=openat2}.args.how

In the last example, the expression {type=syscall}{syscall=openat2} contains two "enum requirements". An enum requirement sets the value for an enum property, unlocking additional properties that are available only for that specific value of the enum: in the aforementioned example,

• the syscall enum property is available under steps only if the enum property type is set to syscall;
• the args property is availaable under steps only if the enum property syscall is set to openat2

The documentation output format is by default a custom textual representation, similar to YAML. The user can select the YAML encoding by using the (--format|-f) yaml flag.

For additional details, see event-generator declarative explain --help.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[poiana]

LGTM label has been added.

Git tree hash: 06f8523811c06e04c0e7838deabc5125364369db

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ekoops, leogr

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