porposal: introduce Commitment method on share.Blob

[Wondertan]

In the node, we have a Blob type that has the commitment cached on the Blob struct as a field. With all the type unification efforts, this seems to be the missing piece to fully migrate over go-square types.

Besides the node needs, it is generally handy to have a method that will lazily compute and cache the commitment on the blob, and users won't need to lookup inclusion pkg for the most common usecase.

[Wondertan]

cc @vgonkivs, @cmwaters, @rootulp

[cmwaters]

Makes sense

[vgonkivs]

Agree with @Wondertan. Can start working on this issue once migration to v2 is done.