vtgate: validate important flags during startup

[harshit-gangal]

Taken from #7251 to get it merged.

1.modify proper annotations for gates
2.add CheckCellFlags function to give valid check before gates creating
3.CheckCellFlags will help to avoid strange behaviors when there are invalid parameters of the cell-related set
vtgate brings up but actually does not work correctly.

Before :
when you are setting cell wrong you will get some confusing error outputs like :

now you will see it a bit clear:

You could also set cell flag correct but cells_to_watch flag with two does not existing cells but vtgate will could also bring up but just will serve not as expectingly. like below:

also if you Accidentally configured -tablet_types_to_wait flag wrongly like below:

also, vtgate will runs but when you are using a command like use @replica
and then you send a query like "select * from table1" it will send qps to all tablets with all types.

CheckCellFlags will help to make an in advance valid check and give hints to make people check and be familiar with the key parameters on cell-related flags

Description

Related Issue(s)

Closes #3955

Checklist

• Tests were added or are not required
• Documentation was added or is not required

[frouioui]

Just a couple of nits! Looking good @Johnny-Three