## Summary of changes
This PR introduces three new abstractions:
1. `Resolver`: Resolves which reader and writer to use for a template.
2. `Writer`: Writes a template project to disk. Prompts the user if
necessary.
3. `Reader`: Reads a template specification from disk, built into the
CLI or from GitHub.
Introducing these abstractions helps decouple reading a template from
writing it. When I tried adding telemetry for the `bundle init` command,
I noticed that the code in `cmd/init.go` was getting convoluted and hard
to test. A future change could have accidentally logged PII when a user
initialised a custom template.
Hedging against that risk is important here because we use a generic
untyped `map<string, string>` representation in the backend to log
telemetry for the `databricks bundle init`. Otherwise, we risk
accidentally breaking our compliance with our centralization
requirements.
### Details
After this PR there are two classes of templates that can be
initialized:
1. A `databricks` template: This could be a builtin template or a
template outside the CLI like mlops-stacks, which is still owned and
managed by Databricks. These templates log their telemetry arguments and
template name.
2. A `custom` template: These are templates created by and managed by
the end user. In these templates we do not log the template name and
args. Instead a generic placeholder string of "custom" is logged in our
telemetry system.
NOTE: The functionality of the `databricks bundle init` command remains
the same after this PR. Only the internal abstractions used are changed.
## Tests
New unit tests. Existing golden and unit tests. Also a fair bit of
manual testing.
shreyas-goenka