Rewrite flagutil.OptionalFlag to use generics

[ajm188]

Now that we have generics, we can stop hand-writing each implementation of flagutil.OptionalFlag for each type we want, instead writing a generic OptionalFlag over any type T, providing a parsing function that has the signature func(string) (T, error)

[MdSahil-oss]

@ajm188 I would like to give a try to this issue.

[divaspathak]

Hello @ajm188, I would like to work on this issue. Can you please assign it to me?

[ajm188]

I believe @MdSahil-oss is working on this, unless that's changed. Please feel free to collaborate with each other in the OSS slack

[SaarthakMaini]

@ajm188 , I would like to work on this issue if no one is working on it
If possible, could it be assigned to me?
Thank you!

[ajm188]

@ajm188 , I would like to work on this issue if no one is working on it
If possible, could it be assigned to me?
Thank you!

#11154 (comment)

[gokulav137]

I would like to have a go at this if thats fine, @ajm188