add --tls-curve-preferences flag to grpc-proxy

[chaitanyabodlapati]

Summary

Adds a new --tls-curve-preferences flag to etcd grpc-proxy, along with
a small cleanup of a shadowing local variable.

Motivation

Go 1.24's native FIPS 140-3 mode (GODEBUG=fips140=only) rejects X25519
during TLS handshake. There is currently no way to restrict the
proxy's curve selection, so FIPS-mode deployments cannot use the
grpc-proxy. This PR adds an opt-in flag that restricts curves.

Behavior

• Default (empty) → Go's default curve selection (unchanged).
• Non-empty → parsed as comma-separated list; unknown names are errors.
• Scope: proxy → backend client TLS only. Matches scope of --tls-min-version.
• Independent of --tls-min-version / --tls-max-version.

Example

etcd grpc-proxy start
--endpoints=https://host:2379
--tls-curve-preferences=P-256,P-384,P-521

Testing

• New unit tests in client/pkg/tlsutil.
• Manual: built, ran with GODEBUG=fips140=only, verified successful
  Without this flag under FIPS mode,
  the handshake fails with crypto/ecdh: use of X25519 is not allowed in FIPS 140-only mode.

Related

• add tls min/max version to grpc proxy #18816 (TLS min/max flags for grpc-proxy, merged).
• Go 1.24 FIPS 140-3 module: https://go.dev/doc/fips140

[k8s-ci-robot]

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This pull-request has been approved by: chaitanyabodlapati
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[k8s-ci-robot]

Hi @chaitanyabodlapati. Thanks for your PR.

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