fix(Constants): fix CDN endpoint typings

[advaith1]

Please describe the changes this PR makes and why it should be merged:
CDN typings had a surprising number of issues so this fixes them:

• DefaultAvatar takes "discriminator" (number), not id
• a few used userId when they were not for users
• added dynamic option to Avatar and Icon (pulled from the banner prs)
• a bunch of id args were typed as Snowflake | number for some reason, changed those to just Snowflake
• some methods actually take format and size in an object (for some reason) but they were typed as separate args

also changes clientId to appId to reflect Discord now using "application id" in non-OAuth2 contexts

Status and versioning classification:

• This PR only includes non-code changes, like changes to documentation, README, etc.