Support ServiceAccountToken in ecr-credential-provider

[fletcherw]

Extend ecr-credential-provider to support fine-grained access via kubernetes ServiceAccount tokens and STS's AssumeRoleWithWebIdentity.

This allows users to avoid long-lived secrets in their pods/nodes and instead use a short-lived credential generated by kubernetes in order to access private ECR images.

What type of PR is this?
/kind feature

What this PR does / why we need it: This PR

Which issue(s) this PR fixes:

Fixes #1147

Does this PR introduce a user-facing change?:

This PR adds support for using ServiceAccount tokens to authenticate with AWS when pulling images from private ECR registries. (See https://github.com/kubernetes/enhancements/issues/4412)
Users who wish to use this new feature will need to opt in to the feature flag and configure their kubernetes cluster and AWS resources appropriately.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: fletcherw / name: Fletcher Woodruff (dd582d2, 623f9e2, 6eb6fae, d853195, 69179ae, db9c696, 36b3df7, cecce90)

[k8s-ci-robot]

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Hi @fletcherw. Thanks for your PR.

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[cartermckinnon]

/ok-to-test

[fletcherw]

/assign cartermckinnon

[cartermckinnon]

This is great, thanks @fletcherw!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cartermckinnon

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