Support additional plugin manifests

[vcheung-stripe]

Reviewers

r? @
cc @stripe/developer-products

Summary

Add support for merging multiple plugin manifests into one plugins.toml, so we can conditionally enable plugin visibility for specific users. For example, to enable the foo plugin, we send to the client:

GET /v1/stripecli/get-plugin-url
{
  "base_url": "https://stripe.jfrog.io/artifactory/stripe-cli-plugins-local",
  "additional_manifests": [
    "plugins-foo.toml"
  ]
}

where plugins-foo.toml contains the manifest for foo, and the client handles merging the data into plugins.toml locally.