feat(Webhook): add 'fetchMessage' method

[izexi]

Ref: discord/discord-api-docs#2812
This PR will be marked as draft until the referenced PR above has been merged.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)

[ThaTiemsz]

has been merged now in discord/discord-api-docs@94d57ea. On a side note, shouldn't the jsdocs/typings for the method also accept a string since @original is also accepted as parameter?

[izexi]

has been merged now in discord/discord-api-docs@94d57ea. On a side note, shouldn't the jsdocs/typings for the method also accept a string since @original is also accepted as parameter?

Although Snowflake is currently typed as a string and passing @original would work, I do agree that it would be nicer to have a union type that also accepts @original as a string literal and or a JSDoc info tag along with that.

I was going to comment on this to be added in #5448 after this PR has been merged, since I think its more within the scope of that PR (and the same will need to be done on Webhook#editMessage/deleteMessage()).