Enable custom flags

[derekperkins]

It should be possible to enable cascading flags for all of the Vitess binaries. Here is an example of how that looks in the current helm charts:

vttablet:

  # Additional flags that will be appended to the vttablet command.
  # The options below are the most commonly adjusted, but any flag can be put here.
  # run vttablet --help to see all available flags
  extraFlags:
    # query server max result size, maximum number of rows allowed to return
    # from vttablet for non-streaming queries.
    queryserver-config-max-result-size: 10000

    # query server query timeout (in seconds), this is the query timeout in vttablet side.
    # If a query takes more than this timeout, it will be killed.
    queryserver-config-query-timeout: 30

https://github.com/vitessio/vitess/blob/master/helm/vitess/values.yaml#L260-L291

Those should be able to be set globally for vttablet, vtgate, etc, with the option to override them at all subsequent levels - keyspace, shard, individual tablet. These flags should be unique, so that more specific values override the more general values.