This change add account_id as part of the identity resolution, from the different credentials provider. It also validates whether an account should have been resolved based on the configure option account_id_endpoint_mode. The way this is done is by using lazy resolvers. We wrap the credentials provider into a custom lazy resolver that will avoid resolving credentials more than once, which means that in that credentials lazy resolver the value will be resolved once, and it will be returned everytime the credentials provider is consumed/invoked. For accountId builts-in, we also use a lazy resolver which holds the validation for wheter account_id value should have been resolved as part of the resolved identity. This accountId built-ins lazy resolver is resolved from endpoint resolution.

For resolving credentials just once per request, and no just once per client initialization, we are adding a middleware that will call forceRefresh method from the CredentialsLazyResolver implementation, and since this middleware will be executed in every request then, the credentials will be fresh per request. We are also fixing here how account_id_endpoint_mode configuration is being resolved.

In this change we add AccountId resolution for when AccountId is part of the rulesetParameters and accountIdEndpointMode is not disabled, in EndpointV2Middleware. If AccountId should be resolved then, we will resolve the credentialsProvider, which will resolve an identity from which we will extract the accountId, and if an accountId is not present in that resolved identity then, we will log a warning if accountIdEndpointMode is set to preferred or throw an exception if accountIdEndpointMode is required. Another part of this change is, that if we get up to the point of resolving the credentials provider then, we inject the resolved identity into $command['@context'] as the property 'resolved_identity'. We do this for trying to avoid having to resolve credentials more than once per request.

Addresses feedback for conditionally calling the method resolvingAccountId based on if the parameter is present in the rulesset of the endpoint provider.
Include tests coverage for AssumeRoleCredentialProvider, AssumeRoleWithWebIdentityCredentialProvider, process, shared files, and credentials from environment.

The validation of whether account_id should be resolved or not is going to be done by the ruleset, and therefore the validation logic around this needs to be removed.

- Move test cases into a valid existent class. In this case the tests in SourceAccountIdEndpointTest were moved to StsClient class.
- Create test for InstanceProfileProvider
- Create test for EcsCredentialProvider

Remove null coalescing operator from the CredentialProvider class in order to prevent Docs Build to fail.

Make endpoint test cases compatible with account_id endpoint mode.

Resolved merge conflicts when merging master into feat_account_id_endpoint_support branch.

Update test account values to be in compliant with our git-secrets checks.
We can use either dummy values, such as `foo`, or the fixed values `123456789012` or `999999999999`.