feat: expose merge strategy for configuration when automerging (#10627)

docs/usage/configuration-options.md

lib/config/definitions.ts

lib/config/types.ts

lib/platform/azure/index.md

lib/platform/bitbucket-server/index.md

lib/platform/bitbucket/__snapshots__/index.spec.ts.snap

lib/platform/bitbucket/index.md

lib/platform/bitbucket/index.spec.ts

lib/platform/bitbucket/index.ts

lib/platform/bitbucket/types.ts

lib/platform/bitbucket/utils.ts

lib/platform/gitea/index.md

lib/platform/github/index.md

lib/platform/gitlab/index.md

lib/platform/types.ts

lib/workers/pr/automerge.ts