feat: create pull requests for solution merges

[ewingjm]

Purpose

Currently, solution merges can only be pushed directly to a branch named 'master'. This is a problem because:

• No pull request means it is possible to accidentally commit changes that would only be visible at the source level
• A fixed target branch of 'master' means difficulty supporting various Git workflows (or alternative default branch names such as 'main')
• Merge conflicts cause a solution merge to fail 'silently' meaning that the next solution merge will contain the failed solution merge's solution components but will be missing the source branch
• Only one work item could be linked to a solution merge commit

Resolves #35, resolves #31, resolves #25.

Approach

Introduces two new configuration tables - Projects and Repositories. When configuring a repository, it's now possible to specify a Source Control Strategy of either Push or Pull request. It's also possible to configure a target branch e.g. 'master', 'main', 'develop' etc.

A source control strategy of Push will behave similarly to older versions of the Development Hub with one exception - the solution merge will only go to a status of Merged when the commit is in source control. Previously, the solution merge would go to a status of Merged when a new Solution Version record was created with the post-merge unmanaged and managed zips attached. Solution versions have been removed as part of this commit in favour of the source control based approach.

A source control strategy of Pull request will go from a status of Merging to a status of Awaiting PR Merge. When a solution merge reaches this status, a pull request will have been automatically created for the solution merge. It will reach a status of Merged when this pull request has been completed.

TODOs

• Automated test coverage for new code
• Documentation updated (if required)
• Build and tests successful