Pull init command's Run method logic into separate method in new file

[SarahFrench]

This PR doesn't change the logic of the Run method on the init command, but it refactors code so that logic is in a separate file.

This is preparation for making non-trivial changes to the init command's Run method for the PSS project. Our intention is to end up with the Run command looking like this:

// internal/command/init.go
func (c *InitCommand) run(args []string) int {
    if c.Meta.AllowExperimentalFeatures {
        // use experimental version of init logic
    } else {
        // use original version of init logic in internal/command/init_run.go
    }
}

There would be the original version of the run logic in internal/command/init_run.go and in future we'd add a newer version of the logic in a yet-to-be-made file. We can then iterate on the newer version of the logic on main branch ok, and when PSS stops being experimental we would update the contents of internal/command/init_run.go.

Target Release

1.14.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[radeksimko]