Fixes Kubernetes Service using expired credentials

[tigrato]

The Kubernetes service occasionally fails to forward requests to EKS clusters or retrieve the cluster schema due to AWS rejecting the request with an "expired token" error.

EKS access tokens are generated using STS presigned URLs, which include details such as the cluster, backend credentials, and assumed roles. By default, these tokens are valid for 15 minutes, and the Kubernetes service refreshes them every $(15 - 1) / 2 = 7\text{ }minutes$. However, our cloud SDK caches the underlying aws.Session, particularly those with assumed roles, for 15 minutes.

This leads to a scenario where the token is refreshed a second time at approximately 14 minutes, close to the token's 15-minute validity. If the underlying credentials expire before the next token refresh, given that they were reused from the previous query and cached since then, it results in the Kubernetes Service considering the token valid (since it is a Base64-encoded presigned URL without knowledge about the credentials), but AWS EKS cluster rejects the request, treating the credentials as expired.

This PR adds an option to disable cache for EKS STS token signing which results in creating a session per EKS cluster sign process.

Bellow one can find the error message EKS returns.

2024-12-09T17:00:15Z ERRO [KUBERNETE] Failed to update cluster schema error:[
ERROR REPORT:
Original Error: *errors.StatusError the server has asked for the client to provide credentials
Stack Trace:
	github.com/gravitational/teleport/lib/kube/proxy/scheme.go:140 github.com/gravitational/teleport/lib/kube/proxy.newClusterSchemaBuilder
	github.com/gravitational/teleport/lib/kube/proxy/cluster_details.go:193 github.com/gravitational/teleport/lib/kube/proxy.newClusterDetails.func1
	runtime/asm_amd64.s:1695 runtime.goexit
User Message: the server has asked for the client to provide credentials] pid:7.1 start_time:2024-12-09T17:00:15Z proxy/cluster_details.go:210
2024-12-09T17:00:24Z ERRO [KUBERNETE] Failed to update cluster schema  error:[
ERROR REPORT:
Original Error: *errors.StatusError the server has asked for the client to provide credentials
Stack Trace:
	github.com/gravitational/teleport/lib/kube/proxy/scheme.go:140 github.com/gravitational/teleport/lib/kube/proxy.newClusterSchemaBuilder
	github.com/gravitational/teleport/lib/kube/proxy/cluster_details.go:193 github.com/gravitational/teleport/lib/kube/proxy.newClusterDetails.func1
	runtime/asm_amd64.s:1695 runtime.goexit
User Message: the server has asked for the client to provide credentials] pid:7.1 start_time:2024-12-09T17:00:24Z proxy/cluster_details.go:210

Changelog: Fixes an intermittent EKS authentication failure when dealing with EKS auto-discovery.

Fixes #50072

[public-teleport-github-review-bot]

@tigrato See the table below for backport results.

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