log/slog: add WithAttrs and Logger.WithAttrs

[mohammadv184]

This merge request adds the WithAttrs function to the log/slog package.

Updates #66937

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Mohammad Abbasi:

Patch Set 3:

(1 comment)

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