[gatsby-image] add placeholderStyle prop to gatsby-image

[jgautsch]

@pieh Per our discussion, this replaces #5653

The original intention is to enable users to add styles to the image placeholder, such as a CSS blur.

For example, a nice blur can now be added with:

<Img
  resolutions={data.file.childImageSharp.resolutions}
  placeholderStyle={{
    filter: `blur(12px)`,
    WebkitFilter: `blur(12px)`,
    MozFilter: `blur(12px)`,
    msFilter: `blur(12px`,
    OFilter: `blur(12px)`
  }}
/>

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[pieh]

Great, thanks!

[jlengstorf]

Hiya @jgautsch! 👋

This is definitely late, but on behalf of the entire Gatsby community, I wanted to say thank you for being here.

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