Support configuring merge strategy for automerge option

[dannysauer]

The digger config file allows setting auto_merge: true, but does not have a setting to configure whether that happens using merge, squash, or rebase. This results in potentially unexpected behavior if a repository allows for multiple methods. In the case of merge and squash, it might also be useful to allow configuring the commit message (or at least the method used to generate the commit message) - but expecting the repo admin to configure the default message in Github's settings (I forget how Gitlab handles that) is probably enough.

This Mergify blog post has a lot of good links. The Github blog post is probably enough information for Github, though.

[goedelsoup-clari]

Running into this in a CDKTF TypeScript monorepo; if I have changes to CDK stacks as well as applications in the same PR then my auto-versioning via conventional commits is broken because Digger not only squashes, but overrides the commit body with auto-merge.

Users can configure the default squash format on the repo level and passing an empty string to the API for commit message will cause the default format to be used.