EuiPageTemplate type issues

[flash1293]

The EuiPageTemplate component has some mistmatched types:

• component property which is not respected
• mainProps doesn't have a component property (but it works when specified)

    <EuiPageTemplate
      component="main" // type checks, but not respected
      mainProps={{ component: 'main' }} // type error, but respected
    />

[cee-chen]

I'm not 100% sure what Caroline's original intentions were with page template but I would assume she would want component="main" to work as a top level prop, so I'll proceed with the bugfix as such (vs. updating the types)