Add license

[libremente]

List of Changes

This PR:

• adds a LICENSE file (MPL-2.0);
• adds licensing info in the package.json file (LMK if this makes sense for you)

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

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Latest commit: 4c1335d

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[libremente]

@lucacavallaro I added LICENSE files and the license key in the various package.json.
The declaration inside package/io-sign is a bit redundant considering that we are already declaring the same license info in the root's package.json, however I would keep it like this since I believe it's better to explicit the LICENSE conditions for each app/package.
If it's OK for you please squash and merge to avoid unnecessary history noise.
Thanks!