pre-load manifests

Say I know in advance that i'm working with a certain cem, like my company's design system.
I want to configure ce-ls to always load that manifest up, by URL

possible config:

```lua
require'lspconfig'.custom_elements_ls.setup {
  preload = {
    cache_expiry = '1w',
    manifests = {
      'https://unpkg.com/@patternfly/elements/custom-elements.json',
    },
  }
}
```