fix(logger): Support .info("foo %s", "bar") formatting

[huonw]

Issue #, if available: N/A

Description of changes:

This adjusts the default logging formatter (LambdaPowertoolsFormatter)
to handle non-structured logs with formatting, like:

logger.info("foo %s", "bar")

This allows the formatter to be used in more situations, such as the
universal formatter for all log messages coming out of a system, even
libraries using the normal logging functionality, without explicit
support for aws-lambda-powertools.

If formatting is performed, this is taken as a hint that the value
probably isn't semantically JSON, even if it's syntactically valid as
JSON.

(I've called this a bug fix in the title but could be a 'feat' too, I guess?)

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

Breaking change checklist

RFC issue #: N/A

• Migration process documented
• Implement warnings (if it can live side by side)

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[heitorlessa]

Hey @huonw thank you so much for this ;)

[huonw]

Thanks for merging so quickly! (And thanks for your efforts on this library, it makes our app noticeably smoother.)