[pkg/stanza] remove all usage of Multiline from the fileconsumer package

[atingchen]

Can we remove all usage of Multiline from the fileconsumer package?

We cannot remove Multiline from the receivers that already depend on it. However, we may be able to have their config look like:

type FileLogConfig {
      fileconsumer.Config `mapstructure:",squash"`
      helper.Multiline    `mapstructure:",squash"`
}

Then what we need to do is identify the necessary interface that allow fileconsumer to work the same as it did before. Essentially, we would make fileconsumer always use "custom" split behavior. What do you think, is it possible?

Originally posted by @djaglowski in #14605 (review)

[github-actions]

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