Add some missing Nullable annotations

Enable built-in Optional converter on Android API 24+

Use a dummy value to further validation optional delegation

Prior to this other things might cause a null value. This ensures the value can only come from only place.

Forbid path traversal ('.' and '..') as @Path parameters.

They're likely to have the unintended effect. For example, passing ".."
to @DELETE /account/book/{isbn}/ yields @DELETE /account/.

Update Proguard rules for R8 fullmode

Taken from here https://github.com/JakeWharton/SdkSearch/commit/498ab309cf83e47e2546379dc1c59eac27711bec

Replace @ParametersAreNonnullByDefault with @EverythingIsNonNull

Fix Optional converter factories not delegating properly.

Move Optional support into Retrofit core.

Test all Rx types can be subscribed to twice

Fix Javadoc formatting.

Move CompletableFuture support into Retrofit core.

Expose static factory for Invocation

Also rework the internals of how an Invocation gets instantiated and set as a tag on the Request. This will make a future change easier to land.

New 'Invocation' class for tracking metrics on Retrofit calls

Normalize null args to empty

Streaming annotation is for ResponseBody.

There is no reason to propagate the pain of nullability throughout multiple internal types.

Add reified create extension for Kotlin

Re-work unresolvable type detection in parameters

In order to natively support Kotlin's 'suspend' modifier in the future, we need to allow wildcards to be used for the last parameter. As a result, we only validate the shape of a type once a Retrofit annotation is detected.

Get rid of the now-useless HttpServiceMethod.Builder

A simple static factory will do.

Update Proguard rules for D8