Add support for Microfrontends

[kitfoster]

Add vercel_microfrontend_group and vercel_microfrontend_group_membership. Microfrontends is currently in private beta.

data "vercel_project" "parent-mfe-project" {
  name = "my parent project"
}

data "vercel_project" "child-mfe-project" {
  name = "my child project"
}

resource "vercel_microfrontend_group" "example-mfe-group" {
  name        = "my mfe"
  default_app = {
    project_id = vercel_project.parent-mfe-project.id
  }
}

resource "vercel_microfrontend_group_membership" "child-mfe-project-mfe-membership" {
  project_id             = vercel_project.child-mfe-project.id
  microfrontend_group_id = vercel_microfrontend_group.example-mfe-group.id
}

[mknichel]

@kitfoster There was a discussion on https://vercel.slack.com/archives/C088GDPK35Y/p1738867552690349?thread_ts=1738866684.654279&cid=C088GDPK35Y about whether we want to have projects be listed as part of the microfrontends group, or separate them so the project resource specifies the microfrontends group ID that it's a part of. Would you mind taking a look and sharing your thoughts?