[vtadmin] Remove explicit Dial-ing for vtctldclient proxy

[ajm188]

Description

Okay!! This is something I've wanted to do since #9977; since, for a given proxy, grpc will keep going back to the discovery to get a fresh address list, there's no need for us to provide Dial/Close semantics and do any sort of manual futzing around ourselves (see the large amount of deletions of if err := vtctld.Dial(...); err != nil this removes for just how much we previously had to care about this).

This sets up a bunch of prep work and then finally changes vtctldclient.New to Dial once on startup. Then, every cluster/api method will assume each vtctld has a connection (or if not, we're in transient failure and grpc will either fix it in time or we fail the request -- this is consistent with current behavior).

The only functional difference is that Close now really, truly, permanently closes a proxy. You need to New up another one, since you can't call Dial on it (the method is now private)

change breakdown

• refactor Dial/Redial tests to operate under the new model
• thread contexts from startup all the way through to vtctldclient.New
• remove all explicit Dial calls
• remove Dial from the interface

testing

• all unit tests pass (with both -count=50 and -race so i'm feeling good)
• can run the local example

Related Issue(s)

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[doeg]

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[github-actions]

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