[vtadmin-api] Reorganize tablet-related functions into vtadmin/cluster/cluster.go

[doeg]

Description

Here's the refactoring I promised to do for #7528 (comment).

Changes:

• Moves everything out of tablets.go into cluster.go so functions have a signature of func (c *Cluster) ...
• Moves getTablets, findTablet, and findNTablets out of api.go and into cluster.go + adds tests
• Moves errors.go out of package vtadmin and into package errors so that it can be shared by api.go + cluster.go

I staged this internally + it works as expected.

Related Issue(s)

• [vtadmin-api] Add /vtexplain endpoint #7528

Checklist

• Should this PR be backported? No
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

N/A

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[ajm188]

I like this separation a lot better; thank you for doing it!! I have a couple small comments but looks great

[doeg]

@ajm188 this is ready for another pass, when you get a sec.