feat: add new keyword for setting locale

Currently, when one would like to set his locale while reusing the `robotframework-faker` library, he will either need to Import the library with locale argument or add it to the settings section.
There is no way currently (AFAIK) to have multiple locales in use on the same file.
So maybe we can add a new keyword to support setting locale option.
@emanlove FYI

Start by inspecting robotframework-faker's existing locale argument and settings handling, then compare how its current keywords are exposed. Done means a keyword can set the locale for use in the same file while preserving support for multiple locales; the issue names no files or tests to run.

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