Implement credential providers

[thekid]

Implements #7 with the following API:

use com\amazon\aws\CredentialProvider;

// Returns credentials from the default credential provider chain, checking, in the following order:
// 
// 1. Environment variables
// 2. Shared credentials and config files
// 3. Amazon ECS container credentials
// 
// If none of the above provide credentials, an exception is raised.
$credentials= CredentialProvider::default()->credentials();

• [x] Environment variables
• [x] Shared credentials and config files
• [x] Amazon ECS container credentials
• [x] Credentials expiration and caching

See https://docs.aws.amazon.com/sdkref/latest/guide/standardized-credentials.html and https://docs.aws.amazon.com/sdk-for-kotlin/latest/developer-guide/credential-providers.html

[thekid]

The full list supported by the PHP SDK is:

• first checks for environment variables,
• then checks for assumed role via web identity,
• then checks for cached SSO credentials from the CLI,
• then check for credential_process in the "default" profile in ~/.aws/credentials,
• then checks for the "default" profile in ~/.aws/credentials,
• then for credential_process in the "default profile" profile in ~/.aws/config,
• then checks for "profile default" profile in ~/.aws/config (which is
• the default profile of AWS CLI),
• then tries to make a GET Request to fetch credentials if ECS environment variable is presented,
• finally checks for EC2 instance profile credentials.

See https://github.com/aws/aws-sdk-php/blob/master/src/Credentials/CredentialProvider.php#L56

---

The missing providers AWS STS web identity (including Amazon Elastic Kubernetes Service (Amazon EKS)) as well as Amazon EC2 Instance Metadata Service and support for SSO and configured process providers can easily be implemented in subsequent releases.

[thekid]

Released in https://github.com/xp-forge/aws/releases/tag/v1.8.0

[thekid]

The missing [...] support for SSO [...] can easily be implemented in subsequent releases.

SSO support is in https://github.com/xp-forge/aws/releases/tag/v2.1.0