Clean up EntryImpl interface (#370)

* Refactor PKI factory and add type checking

This allows for more DRY addition of new PKI types, and stricter
type checking. This also allows for simpler enumeration of
supported PKI formats which will be used in further updates to
simplify the CLI codebase.

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* revamp CLI flags; support different versions for upload

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* Add Alpine Package type

This adds support for the alpine package format used by Alpine Linux,
which is the concatenation of three tgz files (signature, control data,
and then the actual package files).

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* use shaFlag for --artifact-hash

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* change arg type to PKIFormat

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* defer type-specific validation logic to type code (instead of in CLI); also use CliLogger throughout CLI

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* refactor factory code

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* review comments

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* Clean up EntryImpl interface

Make the interface clearer by removing ambiguity around who and when an
entry should have external objects fetched or validated.

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* update pluggable type README

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>