backend/s3: Add support for overriding EC2 Metadata Service endpoint

[gdavison]

For consistency with the AWS provider, add support for overriding the EC2 Metadata Service endpoint during authentication.

• Add parameter ec2_metadata_service_endpoint
• Document the environment variable AWS_EC2_METADATA_SERVICE_ENDPOINT and using the AWS shared configuration file
• Document the deprecated environment variable AWS_METADATA_URL for consistency with the AWS provider

Relates hashicorp/terraform-provider-aws#22825
Relates hashicorp/terraform-provider-aws#22826
Relates hashicorp/aws-sdk-go-base#89
Relates hashicorp/aws-sdk-go-base#90

[github-actions]

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