feat: apollo graphql link and server

Adds experimental ApolloLink and ApolloServer implementations to support GraphQL over RSocket with Apollo GraphQL.

Motivation:

Spring recently added support for GraphQL over RSocket, and we were interested to see if we could provide similar support via rsocket-js. This addition will provide a mechanism for consuming RSocket transport using GraphQL servers via JS clients (browser etc.).

Additionally, these additions provide an interesting alternative to existing Apollo GraphQL subscription links, such as graphql-ws. Existing GraphQL subscription users could now benefit from a potentially increased number of available transport protocol options, and I believe it would be beneficial for these advancements to be more broadly broadcasted and advocated for in the GraphQL community.

Modifications:

added rsocket-graphql-apollo-link package
added rsocket-graphql-apollo-server package
provided examples for using new packages

Result:

See above.

rsocket / rsocket-js