[vtadmin] create+delete shards

[ajm188]

Description

This adds CreateShard and DeleteShards api endpoints to vtadmin. It also enhances (in a gross way) the testutil package to allow us to exercise code from the cluster down through the grpcvtctldserver (rather than stopping at the client interface boundary as we have up to this point), using the in-process localvtctldclient added in #8882

A brief demonstration

❯ curl -X POST -d '{"keyspace": "testks", "shard_name": "-", "include_parent": true}' localhost:14200/api/shards/local
{"result":{"keyspace":{"name":"testks","keyspace":{}},"shard":{"keyspace":"testks","name":"-","shard":{"key_range":{},"is_primary_serving":true}}},"ok":true}%                                                                                                                                  
❯ curl localhost:14200/api/keyspace/local/testks
{"result":{"cluster":{"id":"local","name":"local"},"keyspace":{"name":"testks","keyspace":{}},"shards":{"-":{"keyspace":"testks","name":"-","shard":{"key_range":{},"is_primary_serving":true}}}},"ok":true}%                                                                                   
❯ curl -X DELETE localhost:14200/api/shards/local\?keyspace_shard="testks/-"
{"result":{},"ok":true}%                                                                                                                        
❯ curl localhost:14200/api/keyspace/local/testks
{"result":{"cluster":{"id":"local","name":"local"},"keyspace":{"name":"testks","keyspace":{}}},"ok":true}% 

Related Issue(s)

#8732
#8740

Checklist

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

Deployment Notes

[doeg]

If you get a chance, can you merge in main + push it up? I want to make sure it'll run the new check from #8993. (Kinda funny that it has to be on the branch to run, but I guess I get it!)

[doeg]

Typo: Servver

[ajm188]

the extra v is for .... vtadmin (thanks 😅 )

[doeg]

Is there a need to check for duplicate shards either here or in grpcvtctldclient?

[ajm188]

we can de-dupe here, but it shouldn't matter because it's safe to delete the same shard over and over

[ajm188]

oh i forgot, we de-dupe at the http layer: https://github.com/vitessio/vitess/pull/9012/files#diff-e0e3972839d7d9989a2d71588588ee8b74d574de94295aadc35c9ccccdb04207R72-R74 which i think is Good Enough ™️

[ajm188]

If you get a chance, can you merge in main + push it up?

sorry, just got back to this, doing it now!!