Cleanup ApolloClient.Builder

[martinbonnin]

Prep work for adding more retry related things in ApolloClient.Builder.

This PR:

• moves the networkTransport/subscriptionNetworkTransport creation and checks to ApolloClient.
• makes all Builder functions accept a nullable value to reset the parameter (and consistency as well).
• gives ApolloClient its own copy of Builder (never mutated)

The previous code was problematic in cases like this:

val builder = ApolloClient.Builder().serverUrl("https://example.com")

val apolloClient = builder.build()
builder.serverUrl("https://somethingelse.com")

// customize cache here
val cachingApolloClient = apolloClient1.newBuilder().normalizedCache(...).build()
// Oh no, cachingApolloClient use somethingelse.com instead of example.com

See also #3759

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[martinbonnin]

Breaking ABI change because the parameter is now nullable.

[martinbonnin]

This is implementation detail => not breaking

[BoD]

⚙️