deps(sdkv3): start migration to v3 with new client builder

[Hweinstock]

Problem

Start the transition to sdkv3. This PR is just a start and is still missing a lot of features (see future work).
Originally part of #5940, but decided to split it up.

Solution

• Do not overwrite existing builder to allow for incremental transition to V3.
• A barebones working model, with the telemetry middleware implemented.
• Expansion of testing for client construction.

Future Work

• Implement middleware for other relevant pieces (custom endpoints, logging, etc.). deps(sdkv3): implement logging and endpoint override for client builder.  Hweinstock/aws-toolkit-vscode#4
• Increase testing coverage, specifically with credentials refreshing. (deps(sdkv3): add tests for credentials refresh logic in V3 Client Builder Hweinstock/aws-toolkit-vscode#3)
• Migrate clients. (ssm: refactor(sdkv3): migrate ssm client #6137)
• Reduce boilerplate for client wrappers and have all clients call destroy.
• Avoid recreating clients on each request, use singleton instead.
• replace hand-made mocks with those from sdk mocking library: https://aws.amazon.com/blogs/developer/mocking-modular-aws-sdk-for-javascript-v3-in-unit-tests/

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License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[justinmk3]

While we're here let's rename packages/core/src/test/shared/defaultAwsClientBuilder.test.ts to awsClientBuilder.test.ts

[justinmk3]

The old awsClientBuilder.ts was reluctant to set these fields. But I think setting them here is low-risk and potentially adds useful info:

• if the caller is wrapped in telemetry.xx.run() then these fields will be overridden by the run() context finalizer anyway
• else, these fields give some useful info

todo: Maybe the run() finalizer logic should be smarter and check if there was a SDK failure (these fields)?