Allow pivot editing closures to be async and throwing.

[gwynne]

When using the attach(_:method:on:_:) and attach(_:on:_:) methods of SiblingsProperty, the optional closure provided to allow editing new pivots before they are saved can now optionally be both throws and async.

Closes #581.

Additional changes:

• All versions of the isAttached(), attach(), and detach() methods of SiblingsProperty will now throw errors (specifically cases of the new SiblingsPropertyError enum) instead of calling fatalError() when an unsaved model is encountered.
• The isAttached() method now uses a much faster and more efficient query to check whether a pivot already exists for the given model.

[codecov-commenter]

Codecov Report

Merging #585 (14d4697) into main (ccea982) will increase coverage by 0.35%. The diff coverage is 37.33%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #585 +/- ## ========================================== + Coverage 47.17% 47.52% +0.35% ========================================== Files 106 106 Lines 4689 4734 +45 ========================================== + Hits 2212 2250 +38 - Misses 2477 2484 +7 ``` | [Files Changed](https://app.codecov.io/gh/vapor/fluent-kit/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [Sources/FluentKit/FluentError.swift](https://app.codecov.io/gh/vapor/fluent-kit/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9GbHVlbnRLaXQvRmx1ZW50RXJyb3Iuc3dpZnQ=) | `0.00% <0.00%> (ø)` | | | [Sources/FluentKit/Properties/Siblings.swift](https://app.codecov.io/gh/vapor/fluent-kit/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9GbHVlbnRLaXQvUHJvcGVydGllcy9TaWJsaW5ncy5zd2lmdA==) | `47.09% <11.11%> (+0.75%)` | :arrow_up: | | [...s/FluentKit/Concurrency/Siblings+Concurrency.swift](https://app.codecov.io/gh/vapor/fluent-kit/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9GbHVlbnRLaXQvQ29uY3VycmVuY3kvU2libGluZ3MrQ29uY3VycmVuY3kuc3dpZnQ=) | `55.55% <67.64%> (+55.55%)` | :arrow_up: | | [Sources/XCTFluent/DummyDatabase.swift](https://app.codecov.io/gh/vapor/fluent-kit/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9YQ1RGbHVlbnQvRHVtbXlEYXRhYmFzZS5zd2lmdA==) | `23.77% <100.00%> (+1.08%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/vapor/fluent-kit/pull/585/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor)

[penny-for-vapor[bot]]

These changes are now available in 1.45.0