feat: support customizable timestamp format function

[tisonkun]

Thanks to @kemingy's report in #194, and I was writing a PR as in mosecorg/mosec#680.

Although I think we may have another layout to format in JSON format like what tracing does, it is reasonable to have a customizable timestamp format function here because users' format preferences can be quite different.

See also @nine9ths in #169 where we come with the current default.

[Copilot]

Pull request overview

This PR adds support for customizable timestamp formatting functions in both text and JSON layouts, allowing users to define their own timestamp format instead of being limited to the default ISO 8601 format. The change also includes a refactoring where the timezone field is renamed from tz to timezone for better clarity.

Key changes:

• Added a timestamp_format field to both TextLayout and JsonLayout that accepts a user-defined formatting function
• Renamed the tz field to timezone in both layouts for improved readability
• Extracted the default timestamp formatting logic into a default_timestamp_format function

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 7 comments.

File	Description
layouts/text/src/lib.rs	Added customizable timestamp format function support, renamed timezone field, and refactored timestamp formatting logic
layouts/json/src/lib.rs	Added customizable timestamp format function support, renamed timezone field, simplified serialization by removing custom timestamp serializer

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[tisonkun]

I hope avoiding calling TimeZone::system() or TimeZone::clone() on every logging record could improve performance a bit.