feat: separate build and upload steps (add upload option)

[MohiuddinM]

Description

Right now the cli only has release cmd that builds and uploads together. We should allow both of these steps to be called separately.

Requirements

• add shorebird build command that only builds the artifacts (this cmd takes the same args as the release cmd)
• add shorebird upload command that uploads a previously generated artifact (this cmd takes path of an artifact as arg)

[bryanoltman]

It is intentional that we don't separate these two, as it is often that case that people lose track of which build was made when. What are you hoping to accomplish by splitting these apart?

[MohiuddinM]

We can keep the release command as is, so existing use will not be affected, and people use build and upload separately only when they know what they are doing.

I need this so I can sign the artifact on a separate server.