feat(Integration): Add Integration#roles getter

[vaporoxx]

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

Integration#role currently only returns a single role, however an integration can manage multiple ones. This getter filters the guild's roles by their integration id to return all roles instead.

Consideration

Because the role's tags (see below) aren't guaranteed to be on the object, it's more reliable to use Integration#role. This could be shown in an info/warn tag if Discord is too lazy to fix it before documenting it.

Status

• 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
• Draft PR: This requires feat(Role): role tags #4628 to be merged to make use of the Role#tags property

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.