💡 Add typePrefix support to Contentful source plugin

[ClausR1990]

For ease of use when using multiple instances of the content source plugin. It would be great if you had the option of adding "typePrefix" to avoid conflicting types when sourcing from multiple contentful spaces.

Example:

// In your gatsby-config.js
module.exports = {
  plugins: [
     {
      resolve: "gatsby-source-contentful",
      options: {
        spaceId: "your_space_id",
        // Learn about environment variables: https://gatsby.dev/env-vars
        accessToken: process.env.CONTENTFUL_ACCESS_TOKEN,
      },
    },
    {
      resolve: "gatsby-source-contentful",
      options: {
        spaceId: "second_space_id",
        // Learn about environment variables: https://gatsby.dev/env-vars
        accessToken: process.env.CONTENTFUL_ANOTHER_ACCESS_TOKEN,
        typePrefix: "prefix_"
      },
    },
    "gatsby-plugin-image",
  ],
}