feat: Add guild onboarding settings

[david-why]

Pull Request Type

• Feature addition
• Bugfix
• Documentation update
• Code refactor
• Tests improvement
• CI/CD pipeline enhancement
• Other: [Replace with a description]

Description

Add fetch_onboarding method to Guild objects to allow viewing and modifying guild onboarding settings. Usage example:

onboarding = await guild.fetch_onboarding()
await onboarding.edit(prompts=[], default_channel_ids=[], enabled=False, mode=0)
assert not (onboarding.prompts or onboarding.default_channel_ids or onboarding.enabled)
emoji = PartialEmoji(name=CUSTOM_EMOJI_NAME, id=CUSTOM_EMOJI_ID)
options = [
    OnboardingPromptOption.create('Option 1', role_ids=[ROLE_ID_1], emoji=emoji),
    OnboardingPromptOption.create('Option 2', role_ids=[ROLE_ID_2], emoji='⭐'),
]
prompt = OnboardingPrompt.create(title='Question', options=options, required=True, single_select=True)
await onboarding.edit(prompts=[prompt], default_channel_ids=[CATEGORY_ID], enabled=True, mode=1)
assert (
    onboarding.enabled
    and len(onboarding.prompts) == 1
    and len(onboarding.prompts[0].options) == 2
    and onboarding.prompts[0].title == 'Question'
    and onboarding.prompts[0].required
    and onboarding.prompts[0].single_select
    and onboarding.prompts[0].options[0].title == 'Option 1'
)
await onboarding.edit(enabled=False)

Changes

• Add get_guild_onboarding and modify_guild_onboarding methods to GuildRequests
• Add Onboarding, OnboardingPrompt, and OnboardingPromptOption classes
• Add OnboardingMode and OnboardingPromptType enums
• Add fetch_onboarding method to Guild

Related Issues

#1645

Test Scenarios

Try to edit the onboarding settings for a guild and ensure they are changed (similar to the example given above)

Python Compatibility

• I've ensured my code works on Python 3.10.x
• I've ensured my code works on Python 3.11.x

Checklist

• I've run the pre-commit code linter over all edited files
• I've tested my changes on supported Python versions
• I've added tests for my code, if applicable
• I've updated / added documentation, where applicable

[david-why]

I just rebased from the current interactions-py:unstable branch to keep this PR up-to-date.

[eightween]

Waiting for more approvals before a merge can be done