[sdk-platform-java] Add toString for InstantiatingGrpcChannelProvider

**Is your feature request related to a problem? Please describe.**
When debugging customer issues it is very useful to get a copy of the settings that they are using. Unfortunately since InstantiatingGrpcChannelProvider doesnt implement toString, the settings are hidden from view:

```java
BigtableDataSettings{
// ...
stubSettings=EnhancedBigtableStubSettings{
//....
transportChannelProvider=com.google.api.gax.grpc.InstantiatingGrpcChannelProvider@213ceb4e,
credentialsProvider=GoogleCredentialsProvider{
scopesToApply=[
https://www.googleapis.com/auth/bigtable.data,
https://www.googleapis.com/auth/bigtable.data.readonly,
https://www.googleapis.com/auth/cloud-bigtable.data,
https://www.googleapis.com/auth/cloud-bigtable.data.readonly,
https://www.googleapis.com/auth/cloud-platform,
https://www.googleapis.com/auth/cloud-platform.read-only
],
jwtEnabledScopes=[
https://www.googleapis.com/auth/bigtable.data,
https://www.googleapis.com/auth/cloud-bigtable.data,
https://www.googleapis.com/auth/cloud-platform
],
useJwtAccessWithScope=true,
OAuth2Credentials=null
},
headerProvider=com.google.api.gax.rpc.NoHeaderProvider@23c72128,
internalHeaderProvider=FixedHeaderProvider{
headers={
x-goog-api-client=gl-java/17.0.6__Amazon.com-Inc.__Corretto-17.0.6.10.1 gapic/2.31.0 gax/2.39.0 grpc/1.60.0,
user-agent=bigtable-java/2.31.0,
bigtable-features=CAEgATABOAE=
}
},
....
```

**Describe the solution you'd like**
I would like InstantiatingGrpcChannelProvider to override toString and export its internal config (similar to GoogleCredentialsProvider)

**Describe alternatives you've considered**
I dont think there are any alternatives than to implement this

**Additional context**
Any other context or screenshots about the feature request.