feat: add IAM credentials authentication

[liramon1]

Problem

Flare only provides bare-bones support for IAM credentials. While language clients can provide their IAM credentials to the Flare credentials provider, they must manage IAM credentials themselves. This adds code duplication and complexity to each IDE extension.

Furthermore, the agentic bundle for the CodeWhisperer LSP only supports SSO. If language clients want it to consume IAM credentials, they must start a completely separate bundle which includes the IAM implementation of the CodeWhisperer LSP. This prevents clients from seamlessly switching authentication methods at runtime.

Solution

This feature is split into multiple PRs (in order):

1. feat(identity): add support for IAM profile management #1845
2. feat(identity): add support for retrieving IAM user credentials #1869
3. feat(identity): add STS credential management #1846
4. feat(amazonq): add token methods to codewhisperer clients #1958

Each PR will be merged into feature/flare-iam-base, which will be merged into branch main at the end.

These changes will allow the identity LSP to load and save IAM credentials to the shared config, generate STS credentials from role assumption, and cache/invalidate/refresh STS credentials.

This feature will also contain a major refactor of the CodeWhisperer LSP to switch between consuming IAM and bearer credentials at runtime.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.