refactor(pairing-channel): check the PSK length
Because: * PairingChannelClient.open passed the decoded channel key to the library without a length check. * The TLS 1.3 PSK is one SHA-256 output, so the key must be 32 bytes. This commit: * Makes open throw PairingChannelError INVALID_CONFIGURATION when the decoded key is not 32 bytes. * Changes the test key to 32 bytes and adds tests for keys of 1, 8 and 33 bytes. Closes FXA-13750