Add a `PresignPostObject` to the `PresignClient` for `s3.PutObject` operation

[Madrigal]

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This is used as follows

// conditions that you want your request to have
// see https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html#sigv4-PolicyConditions
conditions := []interface{}{
    []interface{}{
        "content-length-range",
        1,
        10485760,
    },
}
// Duration on which you want this request to
expiresIn = 20 * time.Minute
req, err := presignClient.PresignPostObject(examplePresigner.ctx, &s3.PutObjectInput{
    Bucket: aws.String(bucketName),
    Key:    aws.String(uploadKey),
}, func(opts *s3.PresignPostOptions) {
    opts.Expires = expiresIn
    opts.Conditions = conditions
})
if err != nil {
    log.Printf("Couldn't get a presigned request to put %v:%v. Here's why: %v\n",
        bucketName, uploadKey, err)
}
fmt.Println(req.URL) 
// https://test-bucket.s3.us-west-2.amazonaws.com
fmt.Println(req.Values)
// map[X-Amz-Algorithm:AWS4-HMAC-SHA256
// X-Amz-Credential:ASIAXXXX/20240822/us-west-2/s3/aws4_request
// X-Amz-Date:20240822T215442Z
// X-Amz-Signature:d7c4f16a9a0b8d60a7ed4f01234b0f9c2e58f12d0d3a847f91f1b3e7d6f8192a
// key:my_file.json
// policy:base64encodedValue]

fixes #1224