Add ability to output all keys and values in thread context (v3.0 branch)

[rhanneken]

Description

The keys in a thread context can differ for different code paths. It would be good if one could log whatever keys and values exist in the thread context without specifying all possible keys in the logging configuration.

This change modifies the "context" placeholder so that it can be used without specifying a key. When no key is specified, all keys and their values are sent to the output.

Linked website pull request, if the documention of https://tinylog.org/v3/ has to be updated:

tinylog-org/website#196

Definition of Done

• [x] I read contributing.md
• [x] There are no TODOs left in the code
• [x] Code style follows the tinylog standard
• [x] All classes and methods have Javadoc
• [x] Changes are covered by JUnit tests including edge cases, errors, and exception handling
• [x] Maven build works including compiling, tests, and checks (mvn verify)
• [x] Changes are committed by a verified email address that is assigned to the GitHub account (https://github.com/settings/emails)
• [x] Documentation on the tinylog website is up-to-date or updated by a pull request to the repository tinylog-org/website

Agreements

• [x] I agree that my changes will be published under the terms of the Apache License 2.0 (mandatory)
• [x] I agree that my GitHub user name will be published in the release notes (optional)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.33%. Comparing base (22549cf) to head (38f212d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v3.0 #718 +/- ## ========================================= Coverage 97.32% 97.33% - Complexity 1926 1933 +7 ========================================= Files 170 172 +2 Lines 4152 4163 +11 Branches 400 401 +1 ========================================= + Hits 4041 4052 +11 Misses 54 54 Partials 57 57 ```

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[rhanneken]

What do you think about splitting the logic into two different placeholders? One placeholder for the old logic (outputting the value for a key) and another new placeholder for outputting all thread context entries.

That's a good idea, and I've implemented your suggestion. Let me know what you think.

[rhanneken]

I found just two last minors.

I've addressed them in my latest commit. Let me know what you think.

[pmwmedia]

Thank you for your quick commit. I'm happy to merge the PR :)