Refactor SnippetInputAction to accept actions as symbols

What does this MR do?

In this MR we refactor SnippetInputAction to allow the attribute action as symbols. In the previous version, if the action was a symbol, some validations will fail. Nevertheless, in the upcoming issues, we will receive from GraphQL the action as a symbol and this will help with that (#217795 (closed) and #217799 (closed)).

Besides, we adapt one validation for the update action allowing the attribute previous_path to be nil or ``, which is still a multi-action valid operation.

Does this MR meet the acceptance criteria?

Conformity

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