feat: apollo graphql link and server

[viglucci]

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. 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.

Related:

• https://github.com/apollographql/apollo-server/issues/6264
• https://community.apollographql.com/t/runhttpquery-alternative-for-non-http-frameworks-apolloserver-rsocket-integration/3125

Open questions/items:

• Are we improperly assuming a route will be used/required on the server?
• Are we sending the proper metadata mime type on SETUP and request?
• Are errors being handled according to spec?
• Test Interop with Spring server/client.

[viglucci]

Moving this PR out of draft is pending investigating Apollo's executeOperation, as suggested here.

[viglucci]

I will fix DCO on merge.

[viglucci]

Rebased to fix resolve DCO check issue.