Allow generateKey to be asynchronous and validate the key it returns

New Issue Checklist

• Report security issues confidentially.
• Any contribution is under this license.
• Before posting search existing issues.

Current Limitation

generateKey is called synchronously and its return value is used unchecked:

if (this._generateKey instanceof Function) {
  params.Key = this._bucketPrefix + this._generateKey(filename);
}

Two consequences:

• A key cannot be derived from anything asynchronous. Returning a promise stores the file under a key like test/[object Promise], since the value is concatenated as-is. Anything requiring a lookup, for example resolving a tenant to a folder, cannot be expressed.
• A bad return value fails late and unhelpfully. Returning undefined stores the file under test/undefined. Returning a key longer than the S3 limit of 1024 UTF-8 bytes fails at S3 with an error that does not identify which part of the key was too long. In both cases the upload appears to have been accepted by the adapter.

The generator also only receives the filename, so a key cannot take the content type or the upload options into account.

Feature / Enhancement Description

Await the generator so it may be synchronous or asynchronous, pass it the content type and options, and validate its result before use: a non-empty string whose length including bucketPrefix is within the S3 key limit.

Example Use Case

1. An app stores files per tenant, and the tenant's folder comes from a lookup.
2. generateKey is declared async and resolves the folder before returning the key.
3. The adapter awaits it and stores the object under the resolved key.

Alternatives / Workarounds

Precompute every possible key and hold it in memory, or move naming to the client with preserveFileName, which gives up server-side control of the key.

Start at the generateKey call in the upload path shown in the issue and trace how its return value becomes params.Key. Update the behavior so synchronous and asynchronous generators receive the stated arguments and invalid or oversized keys are rejected before upload; done means the resolved key is used and validation errors identify the adapter failure.