feat(AudioResource): add encoder property, fix type for metadata

[amishshah]

Please describe the changes this PR makes and why it should be merged:

This adds the optional encoder property to an AudioResource, so you could do this for example:

resource.encoder?.setBitrate(48000);

Also simplifies the constructor of AudioResource, as well as fixes the metadata typings. Previously, this was an issue:

(resource as AudioResource<MyMetadata>).metadata // type of MyMetadata|undefined

Even though the AudioResource is given a type, metadata was incorrectly optional. This is now fixed.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• This PR changes the library's interface (methods or parameters added)

[codecov]

Codecov Report

Merging #110 (9c9e8a4) into main (fe3627c) will increase coverage by 0.08%.
The diff coverage is 88.88%.

❗ Current head 9c9e8a4 differs from pull request most recent head 37baa3c. Consider uploading reports for the commit 37baa3c to get more accurate results

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   63.46%   63.55%   +0.08%     
==========================================
  Files          18       18              
  Lines         813      815       +2     
  Branches      191      193       +2     
==========================================
+ Hits          516      518       +2     
  Misses        295      295              
  Partials        2        2              

Impacted Files	Coverage Δ
src/audio/AudioResource.ts	95.55% <88.88%> (+0.20%)	⬆️

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