[gatsby-theme-blog-core] Add canonical to BlogPost interface

[moshfeu]

Why?

This way, the consumer can add canonical to front matters in order to have custom canonical tag. Use case, post original posted on Medium.

I was be able to implement it by modifying the files in node_modules but I couldn't run it through link because I get

error Error in "/Users/moshef/Desktop/themes/packages/gatsby-theme-blog-core/node_modules/gatsby-transformer-sharp/gatsby-node.js": Cannot find module 'at-least-node'
(Tried also to link both gatsby-theme-blog and gatsby-theme-blog-core)

moshfeu@ae50220