Add extension getter for cdn assets

[Lexedia]

Description

Add a way to get the url of the asset without fetching it.

Test are failing because the CI bot doesnt have an avatar, and CdnRequest from nyxx doesn't forward qzery paramters .

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[abitofevrything]

The test could be changed to update the user's avatar to a sample file (there are already test image files in the nyxx repository), and then remove it later/restore it to what it was before.

In fact, what we can do is simply download the current avatar and then re-upload it from bytes. Something like this:

final currentAvatar = (await client.user.get()).avatar;

await client.users.updateCurrentUser(UserUpdateBuilder(
  avatar: ImageBuilder(data: await avatar.fetch(), format: currentAvatar.defaultFormat.extension),
));

This also indirectly serves as a test for updateCurrentUser, though we should probably also add tests for that to the nyxx repository...

[abitofevrything]

Wow, why did GitHub close this? Reopening it...

[Lexedia]

Gotta love gh stopwords

[abitofevrything]

FWIW this is still blocked until the next version of nyxx releases