[Task] Implement utilities for AgentCard signing

[yarolegovich]

Implement utilities for AgentCard signatures.
Signatures are described in the spec and this Python PR can serve as a reference.

1. Create a new package a2acrypto.
2. Make it export a verfifier.

type KeyResolver interface {
  ResolveKey(kid, jku string) (crypto.PublicKey, error)
}

type VerifierConfig struct {
  KeyResolver KeyResolver
}

func NewVerifier(config VerifierConfig) *Verifier { ... }

func (v *Verifier) Verify(card *a2a.AgentCard, signature *a2a.AgentCardSignature) error { ... }

3. Make it export a signer.

type SignerConfig struct {
  PrivateKey crypto.Signer
  KeyID string
  Algorithm string
  JWKSURL string
}

func NewSigner(config SignerConfig) *Signer { ... }

func (s *Signer) Sign(card *a2a.AgentCard) (*a2a.AgentCardSignature, error) { ... }

4. Make a2aclient.Client be configurable with a2aclient.WithCardVerifier(*a2acrypto.Verifier). If set, the verifier should run when GetAgentCard() is called.
5. Make agentcard.Resolver have an *a2acrypto.Verifier field. If set, the verifier should be used in Resolve().
6. Use the signer to implement an a2asrv.NewSignedCardProducer for wrapping a user-provided AgentCardProducer.

type signedAgentCardProducer struct {
  wrapped AgentCardProducer
  signer  *a2asrv.Signer
}

func (p *signedAgentCardProducer) Card(ctx context.Context) (*a2a.AgentCard, error) { ... }

func NewSignedCardProducer(*a2acrypto.Signer, AgentCardProducer) AgentCardProducer { ... }

Add tests to verify the implementation. Can use Python or other SDK to generate an expected signature to compare against.