Avoid using ConcurrentDictionary for channels with few methods

[JamesNK]

GrpcChannel has a ConcurrentDictionary to cache method details. However, if the channel never has more than a handful of methods then ConcurrentDictionary uses a large amount of memory to store just a few instances.

This PR adds a ThreadSafeLookup type that uses a replacing array for a small number of items and upgrades to a dictionary beyond a threashold.

[Copilot]

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/Grpc.Net.Client/Internal/ThreadSafeLookup.cs:24

• The comment should be corrected to 'Looking up a key in an array is as fast as a dictionary for small collections and uses much less memory.'

// Looking up a key in an array is as fast as a dictionary for small collections and uses much less memory. 

[JamesNK]

@BrennanConroy Good feedback. I don't think the type would have broken and caused errors, but it could have gotten into a weird state (using the dictionary but still having some values in the array, duplicate values in the array). I've placed all updates in the lock. And added a comment about the value that's created.

@gfoidl The main goal here is to optimize idle memory usage. Making the array only as big as it needs to be, plus some array allocations while a channel is first used is an ok tradeoff.