[Task] Implement utilities for AgentCard signing

Implement utilities for AgentCard signatures.
Signatures are described [in the spec](https://a2a-protocol.org/v0.3.0/specification/#555-agentinterface-object) and this Python [PR](https://github.com/a2aproject/a2a-python/pull/581) can serve as a reference.

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