feat: Add default merge method

[ricoberger]

Describe what this PR does / why we need it

This commit adds a new option default_merge_method, which can be used to define the default merge method for the Octo pr merge command.

Until now the Octo pr merge was always using commit when no argument was defined. This can now be customized by the user. A user can select between commit, rebase and squash. If another string is provided it will fallback to commit.

This is especially useful (at least for me 😅) when the enable_builtin option is enabled and the merge command is selected via Telescope where it isn't possible to provide the merge method.

Does this pull request fix one issue?

NONE

Special notes for reviews

• I'm not very familiar with Lua and writing Neovim plugins, so maybe this PR is completely shit
• If this isn't a feature you want to add, feel free to just close the PR (sry I was a bit lazy with creating an GitHub issue first to discuss this)

[pwntester]

Looks great, thanks for the contribution!