Use specified avatar_url in git-gateway backend (1 line fix I think)

[loteoo]

The git-gateway backend doesn't seem to pass the avatar_url field to the user object. A simple fix seems possible.

The avatar_url is received here: https://github.com/decaporg/decap-cms/blob/main/packages/decap-cms-backend-git-gateway/src/implementation.ts#L333

But then the returned object gets rid of it for no reason:
https://github.com/decaporg/decap-cms/blob/main/packages/decap-cms-backend-git-gateway/src/implementation.ts#L365

Describe the solution you'd like
It looks like simply adding avatar_url: userData.avatar_url in that object would fix the issue.

Possibly, I think the type here would also need it an update to add the avatar_url field: https://github.com/decaporg/decap-cms/blob/main/packages/decap-cms-lib-util/src/implementation.ts#L89

Thanks!

[loteoo]

Made this PR: #7247