feat: present a client certificate for mutual TLS

[DavidCozens]

Add a client certificate and its key to the stream config. The handshake then authenticates the
device to the collector, as well as the collector to the device.

struct SolidSyslogMbedTlsStreamConfig tlsConfig = {
    /* ... as the previous TLS stage ... */
    .ClientCertChain = DeviceCertStore_ClientChain(),
    .ClientKey       = DeviceCertStore_ClientKey(),
};

Both fields must be set. Supplying one and not the other leaves the connection
server-authenticated and does not fail, so the pipeline element is given what the device holds
rather than what was configured:

s_sd[3] = SyslogPipelineSd_Init((clientChain != NULL) && (clientKey != NULL));

... [logPipeline@32473 transport="mtls" atRest="hmac-sha256"] device started

The handshake authenticates the TLS peer. Where a relay, gateway or broker terminates the
connection, the collector authenticates that hop rather than the device behind it, and the origin
element carries the device's own identity across it.

The collector port used here requires a client certificate and refuses a client that presents none.

When you need it. When the receiver has to authenticate the device rather than accept the
identity the record claims. It requires a certificate per device, protected storage for the private
key, and an issuing and revocation process behind both.

[DavidCozens]

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