[SRA] Update service clients to remove usage of fallback credentials factory

[muhammad-othman]

Description

• Remove the usage of FallbackCredentialFactory in service client constructors (limited to AutoScaling service for now to avoid huge merge conflicts later).
• Rename AmazonServiceClient.Credentials to be DefaultAWSCredentials.
• Added new IdentityResolverConfiguration property to the client config.
• Marked the ClientConfig.AWSTokenProvider property as obsolete.

TODO: Remove the creation of the Signer property (will work on this part while updating the signer handler).

Motivation and Context

DOTNET-7538

[dscpinheiro]

Suggested change

	/// Gets the configuration for identity resolvers used by the service client, which manages
	/// the resolvers used to obtain various types of identities.
	/// Gets the configuration for the identity resolvers configured in the service client.

I think the documentation could be improved further, but I don't have a better suggestion at the moment (looks like Java ended up not using *Configuration at all: aws/aws-sdk-java-v2#4423).

[muhammad-othman]

I did see that and it took me a while to figure out that the IdentityResolvers is the IdentityResolverConfiguration mentioned in the SRA, so I would prefer to use the SRA names to make it easier to find what it means in the SRA unless we have a good reason not to do so.

[dscpinheiro]

Oh, for sure. I wasn't suggesting renaming the type, just mentioning we can improve the documentation for this property (since it's visible in the config and may not be clear to customers at first).