context variables validation - $(tasks.dagtask.status)

[pritidesai]

Context variable to access execution status is supported as a param value without any other params or extra string. This validation is missing from the PR #3390 that it implemented this feature. PR #3390 has documented the guidelines in pipelines.md:

`$(tasks.<pipelineTask>.status)` is instantiated and available at runtime and must be used as a param value
as is without concatenating it with any other param or string, for example, this kind of usage is not validated/supported
`task status is $(tasks.<pipelineTask>.status)`. 

Use it as a param value:
     params:
        - name: task1Status
          value: "$(tasks.task1.status)"

First, lets evaluate whether we should enforce this restriction, i.e. $(tasks.dagtask.status) can not be used with any other param or concatenated with other string.

To apply such enforcement, add validation for:

1. A dag task with a param value set to $(tasks.dagtask.status) with or without any other param or string is prohibited (part of this validation is implemented in PR access execution status of any task in finally #3390 which includes validating dag task with param value set to $(tasks.dagtask.status) without any extra concatenation).
2. A finally task with param value referring to $(tasks.dagtask.status) must not have any other params or string around it.

PR #3390 has implemented validating existence of dagtask in $(tasks.dagtask.status). This issue only focuses on validation (1) and (2).