Regenerate data-poster external-signer test certs

[PlasmaPower]

The data poster external signer tests were failing because the certificates had expired:

=== RUN   TestExternalSigner/blob_transaction
INFO [10-15|22:26:35.148] http: TLS handshake error from 127.0.0.1:34002: remote error: tls: bad certificate
    dataposter_test.go:108: Error signing transaction with external signer: making signing request to external signer: Post "https://localhost:34719": tls: failed to verify certificate: x509: certificate has expired or is not yet valid: current time 2024-10-15T22:26:35-05:00 is after 2024-10-15T14:40:05Z

This PR adds a shell script to regenerate them, and does so with 100 years until expiry (as these are just test certificates with published private keys, obviously they shouldn't be used for anything except automated unit tests with no funds at risk).

[joshuacolvin0]

LGTM