fix(pricing): honor AWS profile and reuse resolved credentials

[cesar32az]

Description

Fixes #477.

When the AWS SDK's default credential chain can't resolve usable credentials for the active profile, pricing lookups fall back to EC2 IMDS and fail on a laptop:

updating on-demand pricing, operation error Pricing: GetProducts, ...
get credentials: failed to refresh cached credentials, no EC2 IMDS role found,
operation error ec2imds: GetMetadata, ... dial tcp 169.254.169.254:80: connect: host is down

This is common with AWS SSO / IAM Identity Center: kubectl works because its kubeconfig exec plugin (aws eks get-token) resolves its own profile, but eks-node-viewer loads a separate SDK config that has no way to select a profile, so it ends at IMDS.

There were two related gaps:

1. There was no way to tell eks-node-viewer which AWS profile to use — only the ambient AWS_PROFILE env var.
2. NewPricingClient built a brand-new config via LoadDefaultConfig instead of reusing the credentials already resolved for the EC2 client, so even a correctly-resolved config wasn't propagated to the Pricing API client.

Changes

• Add a --profile flag (and a profile key for the ~/.eks-node-viewer config file) to select the AWS profile used for pricing/EC2 calls. When empty, the standard credential chain is used (AWS_PROFILE, shared config, etc.), so existing behavior is unchanged.
• NewPricingClient now copies the already-resolved aws.Config and only overrides the region to a pricing-API-capable one, so the configured profile / SSO session is honored consistently across the EC2 and Pricing clients.
• Remove the no-op config.WithSharedConfigProfile("") in main.
• Document the flag and add a troubleshooting entry for the IMDS error in the README.

Testing

• go build ./cmd/... ./pkg/..., go vet, go test ./cmd/... ./pkg/... all pass. Added pkg/aws/pricing_test.go covering the region mapping and that NewPricingClient doesn't mutate the caller's config.
• Ran the built binary against a live EKS cluster with --profile <name>: pricing updated from the static baseline to live values with no IMDS/credential errors in the logs.
• Verified --profile takes precedence over a bogus AWS_PROFILE env var, and that the flag is rejected by the released binary (confirming it's net-new).

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

[cesar32az]

Closing in favor of #383, which already addresses AWS profile/region resolution for pricing (deriving them from the kube context). Apologies for the duplicate.