feat(gatsby-plugin-sharp): Add image plugin defaults

[ascorbic]

Adds support for users to specify default values for most gatsby-plugin-image options. These are set on gatsby-plugin-sharp, in the defaults key.

e.g.

module.exports = {
  siteMetadata: {
    title: `Gatsby Plugin Image`,
  },
  plugins: [
    {
      resolve: `gatsby-plugin-sharp`,
      options: {
        defaults: {
          breakpoints: [100, 200, 300, 400],
          formats: ["auto", "webp", "avif"],
          transformOptions: {
            fit: "contain",
          },
        },
      },
    },
    `gatsby-transformer-sharp`,
    { resolve: `gatsby-source-filesystem`, options: { path: "./src/images/" } },
    `gatsby-plugin-image`,
  ],
}

[ch23484]

[laurieontech]

One question I have with this is whether it's more intuitive to pass these defaults to gatsby-plugin-image and let them pass on to sharp. I know that's not the plugin in charge of processing, but I expect that's how users will think about it?

[laurieontech]

is jpeg valid?

[ascorbic]

No

[laurieontech]

transformOptions? Any of the sizing stuff like aspectRatio?

[ascorbic]

I don't think the sizing options make sense to have as global defaults. I guess transformOptions could though. I'll add them.

[laurieontech]

Ya, my one thought was aspectRatio could, since it's more look and feel that strict sizing. But no strong feelings on that one.