feat: new datasource spot datafeed subscription

[stefanfreitag]

Description

From #39259: This is a data source that mirrors the aws_spot_datafeed_subscription resource. Every AWS account is allowed up to one spot data-feed subscription, which updates an s3 bucket with the latest EC2 spot instance prices.

Relations

Closes #39259

References

Copied from #39259:

• AWS Go SDK v2: DescribeSpotDatafeedSubscription
• awscli2: describe-spot-datafeed-subscription*
• AWS EC2 API Reference: DescribeSpotDatafeedSubscription
• AWS EC2 User Guide: Track your Spot Instance costs using the Spot Instance data feed
• hashicorp/aws docs: resource "aws_spot_datafeed_subscription"

Output from Acceptance Testing

❯ make testacc TESTS=TestAccEC2SpotDataFeedSubscriptionDataSource_basic PKG=ec2
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.23.1 test ./internal/service/ec2/... -v -count 1 -parallel 20 -run='TestAccEC2SpotDataFeedSubscriptionDataSource_basic'  -timeout 360m
=== RUN   TestAccEC2SpotDataFeedSubscriptionDataSource_basic
=== PAUSE TestAccEC2SpotDataFeedSubscriptionDataSource_basic
=== CONT  TestAccEC2SpotDataFeedSubscriptionDataSource_basic
--- PASS: TestAccEC2SpotDataFeedSubscriptionDataSource_basic (39.80s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/ec2        39.947s
...