logger: add mapping support

[artemklevtsov]

Hello,

I would like to propose adding a new mapping field to the logger component configuration, similar to the existing functionality in metrics (see Metric Mapping documentation).

Motivation:

• Enable dynamic construction of log fields.
• Allow conditional formatting and filtering of log messages.

Request:
Add a mapping configuration option in logger that supports bloblang.

Use case:
Log format with GELF.

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Regards