Gov2: Add presign post request support - S3

[Madrigal]

This is Luis from the Go SDK team

We recently added support for s3 presign POST requests at aws/aws-sdk-go-v2#1224

We'd like to expand the existing examples to add presign post requests.

Let me know if there's anything I should change about the tags used to create the examples so they display correctly at https://docs.aws.amazon.com/AmazonS3/latest/userguide/example_s3_Scenario_PresignedUrl_section.html

This scenario has a bit more complex way of sending the request since multipart requests require more setup. However, this is mostly intended to be used by a browser sending a form rather than sending this directly from a Go server.

Tested this by

1. Running go run ./cmd -scenario presigning and making sure it works as expected with a local file

 go run ./cmd -scenario presigning
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Welcome to the Amazon S3 presigning demo.
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We'll need a bucket. Enter a name for a bucket you own or one you want to create:
lmadrig-presign
Bucket lmadrig-presign exists and you already own it.
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Let's presign a request to upload a file to your bucket.
Enter the path to a file you want to upload:
/Volumes/workplace/sample.txt
What would you like to name the uploaded object?
my-presign
Got a presigned PUT request to URL:
	https://lmadrig-presign.s3.us-west-2.amazonaws.com/my-presign?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=ASIAExample%2F20240927%2Fus-west-2%2Fs3%2Faws4_request&X-Amz-Date=20240927T170511Z&X-Amz-Expires=60&X-Amz-Security-Token=XXXX&X-Amz-SignedHeaders=host&x-id=PutObject&X-Amz-Signature=YYYY
Using net/http to send the request...
PUT object my-presign with presigned URL returned 200.
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Let's presign a request to download the object.
Press Enter when you're ready.

Got a presigned GET request to URL:
	https://lmadrig-presign.s3.us-west-2.amazonaws.com/my-presign?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=ASIAExample%2F20240927%2Fus-west-2%2Fs3%2Faws4_request&X-Amz-Date=20240927T170513Z&X-Amz-Expires=60&X-Amz-Security-Token=XXXX&X-Amz-SignedHeaders=host&x-id=GetObject&X-Amz-Signature=YYYY
Using net/http to send the request...
GET object my-presign with presigned URL returned 200.
Downloaded 12 bytes. Here are the first 100 of them:
----------------------------------------------------------------------------------------
Hello world

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Now we'll create a new request to put the same object using a presigned post request
Got a presigned post request to url https://lmadrig-presign.s3.us-west-2.amazonaws.com with values map[X-Amz-Algorithm:AWS4-HMAC-SHA256 X-Amz-Credential:ASIAExample/20240927/us-west-2/s3/aws4_request X-Amz-Date:20240927T170513Z X-Amz-Security-Token:XXXX key:my-presign policy:base64str]
Using net/http multipart to send the request...
Presign post object my-presign with presigned URL returned 204.
Let's presign a request to delete the object.
Press Enter when you're ready.

Got a presigned DELETE request to URL:
	https://lmadrig-presign.s3.us-west-2.amazonaws.com/my-presign?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=ASIAExample%2F20240927%2Fus-west-2%2Fs3%2Faws4_request&X-Amz-Date=20240927T170516Z&X-Amz-Expires=900&X-Amz-Security-Token=XXXX&X-Amz-SignedHeaders=host&x-id=DeleteObject&X-Amz-Signature=YYYY
Using net/http to send the request...
DELETE object my-presign with presigned URL returned 204.
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Thanks for watching!
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1. Running go test inside the s3 folder

go test ./...
?   	github.com/awsdocs/aws-doc-sdk-examples/gov2/s3/cmd	[no test files]
?   	github.com/awsdocs/aws-doc-sdk-examples/gov2/s3/hello	[no test files]
?   	github.com/awsdocs/aws-doc-sdk-examples/gov2/s3/stubs	[no test files]
ok  	github.com/awsdocs/aws-doc-sdk-examples/gov2/s3/actions	(cached)
ok  	github.com/awsdocs/aws-doc-sdk-examples/gov2/s3/scenarios	(cached)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.