[plugins] Externalize SofaDistanceGrid plugin

[hugtalbot]

Now available here :arrow_right: SofaDistanceGrid

Related to https://github.com/sofa-framework/sofa/issues/1442

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I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[bakpaul]

Will require a PR to https://github.com/sofa-framework/ci to fetch/activate it for the right configuration