[1.0] Make mediaType optional

[KyleAMathews]

When designing Gatsby's data system I originally thought all nodes
should have a media type as a way of signaling to transformer plugins
how they could transform its data.

But while writing more source/transformer plugins and sites I've realized
that most nodes are at a terminal/leaf state, full decomposed with no more need of
transformation. For these nodes, they have no need to set a mediaType because
they're already 100% primitive data types.

So setting mediaType is now opt-in and should only be used when a node
has unprocessed raw data that they don't want to take responsibility for processing
themselves futher.

A common example is markdown. Markdown is commonly used for building websites
and is a supported input type for many CMSs or other online content creation
services.

A source plugin for one of these services will generally want to create nodes
for the markdown and set the mediaType to text/x-markdown so Gatsby transformer
plugins that understand Markdown can then handle converting the markdown into
other formats.

But the markdown transformer plugin, when it creates transformed nodes, won't want
to set a mediaType as the nodes it creates will now be fully decomposed into
transformed HTML, frontmatter fields, etc.

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