Query parameter to handle Filter and Pattern syntax

[dav009]

These changes might fall outside Segmentio's use case, but they have been useful to me:

• Query param, allowing to pass Cloudwatch Filter and Pattern syntax

example:

cwlogs fetch --query {$.status!=200} -f my-group

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• Messages which cannot be unmarshalled following segmentio's schema (ecslogs.Event) are handled as raw text, thus showing some message instead of an empty string.