fix(ec2): properly paginate instances in quickPick.

[Hweinstock]

Problem

Follow up to discussion: #6672 (comment)

Currently the ec2 module does make paginated requests to the SDK endpoint. However, in order to show these results into the UI, we resolve them in entirety.
For the quick picks this is done here: https://github.com/aws/aws-toolkit-vscode/blob/4e568970e8d8646cf8e590d39d8891d68cebde84/packages/core/src/awsService/ec2/prompter.ts#L65
For the explorer this is done here: https://github.com/aws/aws-toolkit-vscode/blob/4e568970e8d8646cf8e590d39d8891d68cebde84/packages/core/src/awsService/ec2/explorer/ec2ParentNode.ts#L52

Solution

• Refactor the EC2 client to process the instances within the pages they come in (i.e. not flattening them). This allows passing an AsyncCollection of pages to the quickPick.
• Avoid flattening results in makePaginatedRequest by default.
• Refactor client builder to be non-async, this causes some trickle-down refactoring, but allows makePaginatedRequest to not return a promise.
• Simplify instance processing to make use of type predicates.
• Remove mocks from Ec2Prompter tests, and refactor to test pagination behavior.

Alternative Solution

• Repack the instances into artificial pages (i.e. a batched iterator). This is a little hacky since it involves us unpacking and repacking the instances. However, it does involve significantly less code changes.

Verification

In addition to updating the tests, we can add a logging statement when the quickPick loads an item from the AsyncIterable. This screenshot shows that loading two items was successful and triggered the AsyncIterable twice, as expected.

Future Work

• Extend this work to the explorer. There is already some general work done in https://github.com/aws/aws-toolkit-vscode/blob/df810aa4264ba14dac488697d539011a732c7eff/packages/core/src/awsexplorer/childNodeLoader.ts, but it doesn't support working with AsyncCollection, only making the requests directly. We could do something hacky, like wrap the AsyncCollection in a fake request method, or try to refactor the utility itself.

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[github-actions]

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[justinmk3]

would we have some other class for "unsafe"? or is that just an improvement that we are making in our wrapper class?

Suggested change

	export interface PatchedEc2Instance extends Instance {
	export interface Ec2Instance extends Instance {

[Hweinstock]

This is an improvement here to make processing the responses easier. Avoids undefined checks in multiple places since we assert it once, and store that info in the type.