Capella LightClientUpdate type is wrong

[aeryz]

LightClientUpdate of capella is a re-export from altair.

But in the spec, although the definition of the LightClientUpdate seem to be unchanged at the top level, the definition of LightClientHeader is changed.
See: altair, capella.

So I am suggesting to add LightClientHeader type to capella and change LightClientUpdate type. I can tackle the PR if I am not mistaken.

[ralexstokes]

hey! thanks for raising this issue

this part of this repo has not been updated since some things in the spec have changed and I'd love to have a PR updating them :)

yeah it looks like there is this new defn of a LightClientHeader and that ripples to update the LightClientUpdate type in capella

if you make a pass on it, im happy to review the PR