Add a foundation for built-in templates

[lennartkats-db]

Changes

This pull request extends the templating support in preparation of a new, default template (WIP, #686):

• builtin templates that can be initialized using e.g. databricks bundle init default-python
• builtin templates are embedded into the executable using go's embed functionality, making sure they're co-versioned with the CLI
• new helpers to get the workspace name, current user name, etc. help craft a complete template
• (not enabled yet) when the user types databricks bundle init they can interactively select the default-python template

And makes two tangentially related changes:

• IsServicePrincipal now uses the "users" API rather than the "principals" API, since the latter is too slow for our purposes.
• mode: prod no longer requires the 'target.prod.git' setting. It's hard to set that from a template. (Pieter is planning an overhaul of warnings support; this would be one of the first warnings we show.)

The actual default-python template is maintained in a separate PR: #686

Tests

Unit tests, manual testing

[pietern]

We should do something else. This can fail for other reasons and if it does will incorrectly assume the user is a service principal. If I look at the scim/me output (that we include in workspace.current_user), there are other heuristics that should be more reliable. Example of workspace.current_user for a service principal:

{
  "active":true,
  "displayName":"sdk-1693297374999987000",
  "emails": [
    {
      "primary":true,
      "type":"work",
      "value":"a1e691b7-a6df-4a6b-881e-d93a03a7ce52"
    }
  ],
  "id":"6079497891687986",
  "name": {
    "givenName":"sdk-1693297374999987000"
  },
  "userName":"a1e691b7-a6df-4a6b-881e-d93a03a7ce52"
}

[lennartkats-db]

Is that always the case though, that the username is an id like that? And will that be the case in the future?

[lennartkats-db]

Actually — this should have error handling code. Not sure what happened to it. Let me add it back in a cleanup PR: #707.