feat: add labels and assignees to the resulting combined-pr

[GrantBirki]

This pull request adds logic for adding labels and assignees to the resulting PR that gets created by this CLI.

Note: I did an experiment and used the agentic mode of copilot to generate these changes and have review/tested them myself.

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Resolves: #10

[Copilot]

Pull Request Overview

This pull request adds functionality for attaching labels and assignees to the combined pull request created by the CLI. Key changes include wrapping the REST client to conform to a new interface, updating functions to use the RESTClientInterface, and extending the pull request creation logic to include labels and assignees.

Reviewed Changes

Copilot reviewed 32 out of 33 changed files in this pull request and generated no comments.

File	Description
internal/cmd/root.go	Introduces a wrapper for the REST client to meet the new interface.
internal/cmd/mock_restclient.go	Implements a mock REST client matching the updated interface.
internal/cmd/combine_prs_test.go	Adds tests for pull request creation with labels and assignees.
internal/cmd/combine_prs.go	Refactors functions to use RESTClientInterface and updates createPullRequest to accept labels and assignees.

Files not reviewed (1)

• go.mod: Language not supported

Comments suppressed due to low confidence (1)

internal/cmd/combine_prs.go:98

• The variables 'addLabels' and 'addAssignees' are used in the createPullRequest call but their definitions are not visible in this diff. Please ensure these variables are properly defined and initialized for the call to succeed.

err = createPullRequest(ctx, restClient, repo, prTitle, combineBranchName, repoDefaultBranch, prBody, addLabels, addAssignees)