ECA check for PRs

Hi,

@vince-fugnitto realized we have the option to install the “Eclipse ECA Validation” check on our repos. Presumably it will become mandatory when we move to an Eclipse Foundation org, but in the meantime it will save time, not having to manually verify the ECA for contributions.

We have gone-ahead and enabled it for the main repo; if it causes issues we can remove it.

see: https://github.com/theia-ide/theia/issues/4641

[original thread by Marc Dumais]