gatsby-source-contentful without dependency on gatsby-plugin-sharp

[just-boris]

Summary

Currently, it is not possible to install Contentful source without getting Sharp plugin. This introduces a few issues

• When Sharp releases are not available, gatsby installation fails. Recent example – https://www.githubstatus.com/incidents/1p369ttzrnqy We were not able to build our website during the incident.
• gatsby-plugin-sharp package depends on packages with tricky licensing – Incompatible Licenses Apache2.0 & GPLv2 (gatsby-plugin-sharp) #17584. Since it is a required dependency of gatsby-source-contentful, this package itself has licensing issues.
• Sharp is a native module, requires additional setup for non-mainstream OS.

The issues above are very frustrating when you don't have any images, so you have to deal with the issues for nothing.

Proposed change

Move gatsby-plugin-sharp to optionalDependencies of gatsby-source-contentful. The change is backward-compatible, the package will be installed anyway, however, if the installation of this module fails, the build will proceed anyway.