feat: add type parameter to execCmd

[mdonnalley]

const command = `auth:jwt:grant -d -u ${username} -i ${clientId} -f ${jwtKey} -r ${instanceUrl} --json`;
const json = execCmd<AuthFields>(command, { ensureExitCode: 0 }).jsonOutput;

In that example the type of json would be:

{
    status: number;
    result: AuthFields;
}

@W-8856588@

[shetzel]

This is all great! I'm requesting changes though to add unit tests.