Add LibreTranslate machine translator, documentation and its test cases.

[drivard]

This PR introduces a new machine translator to the wagtail-localize project, leveraging the LibreTranslate API. The LibreTranslator class extends the BaseMachineTranslator and implements the necessary methods for translation.

It supports configuration alike

WAGTAILLOCALIZE_MACHINE_TRANSLATOR = {
    "CLASS": "wagtail_localize.machine_translators.libretranslate.LibreTranslator",
    "OPTIONS": {
        "LIBRETRANSLATE_URL": "https://libretranslate.org",
        "API_KEY": "<Your LibreTranslate api key here>",
    },
}

The LibreTranslate API can also be hosted using many methods documented on their github page https://github.com/LibreTranslate/LibreTranslate.
I tested it against a self-hosted instance.

Here are some screenshots of an about us page that I translated using the LibreTranslator.

Please review and provide any feedback.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a3efa8f) 92.60% compared to head (8d0d62b) 92.53%.

Files	Patch %	Lines
...ail_localize/machine_translators/libretranslate.py	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #753      +/-   ##
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- Coverage   92.60%   92.53%   -0.07%     
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  Files          46       47       +1     
  Lines        4017     4034      +17     
  Branches      598      600       +2     
==========================================
+ Hits         3720     3733      +13     
- Misses        175      179       +4     
  Partials      122      122              

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[zerolab]

@drivard thank you for this.

Left a few suggestions for improvement

[zerolab]

rather than making an actual API call, I'd test 2 things:

1. that translate will raise for status (so, mock the post call to that effect)
2. that we get a dict of string: StringValue(translation) from the response, if there are translated strings. Again, using a mocked response

[drivard]

could I have a bit of pointer on mocking a post request please?

[zerolab]

Suggested change

	"API_KEY": "<Your LibreTranslate api key here>", # Optional on self-hosted instance by providing a random string
	"API_KEY": "<Your LibreTranslate api key here>", # Use any random string with self-hosted instances.

[drivard]

Maybe something like this would be more appropriate
# On self-hosted instances, if you did not setup the api keys support use any random string.

[drivard]

@zerolab I have completed the changes requests. Please review.

[zerolab]

Thank you @drivard, merged in a458ed8

[drivard]

@zerolab Do you have an idea of when it will be released ?

[zerolab]

Got a couple of more PRs and if all goes well, sometime next week (i.e before the end of the year)