feat(go): add EVM facilitator signer helper

[GrapeInTheTree]

Description

Add NewFacilitatorSignerFromPrivateKey helper to go/signers/evm, implementing the FacilitatorEvmSigner interface.

The Go SDK currently provides a client signer helper (NewClientSignerFromPrivateKey) but no facilitator equivalent — the signers README lists it as "planned". This PR delivers that missing helper.

What it provides:

• NewFacilitatorSignerFromPrivateKey(privateKeyHex, rpcURL) — one-line facilitator signer creation
• All 9 methods of FacilitatorEvmSigner: GetAddresses, ReadContract, VerifyTypedData, WriteContract, SendTransaction, WaitForTransactionReceipt, GetBalance, GetChainID, GetCode
• EIP-1559 dynamic fee transactions with gas estimation + 300k fallback
• EIP-712 signature verification via ecrecover
• Private key stored as fixed-size [32]byte, zeroed on Close()

Background:
Built while developing a self-hosted x402 facilitator on Base Sepolia with real USDC transfers (EIP-3009 and Permit2). Without this helper, every Go facilitator requires ~300 lines of RPC boilerplate.

Tests

cd go && go test ./signers/evm/ -v -count=1

8 new tests added, all 12 tests pass (4 existing client + 8 new facilitator):

• Invalid key handling (empty, not hex, too short)
• Address derivation with known Hardhat test key
• 0x prefix handling
• GetAddresses returns correct address
• Close() zeroes private key bytes
• VerifyTypedData with valid EIP-712 signature
• VerifyTypedData rejects wrong address
• Compile-time interface implementation check

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• I added a changelog fragment for user-facing changes (docs-only changes can skip)

Note: Happy to add a changelog fragment and README updates if reviewers want them in this PR.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[vercel]

@GrapeInTheTree is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.