feat(commands): support bot-wide defaults for install_types/contexts

[shiftinv]

Summary

One of the promised #1173 followup PRs.
The default_install_types and default_contexts parameters in the Bot constructor apply to all application commands that don't have their own value set for those fields (through the decorator/parameter, x_command_attrs, or the GuildCommandInteraction annotation).

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)