[server] introduce execution hooks

[dariuszkuc]

Was just thinking - it seems like the major feature that this PR brings is the ability to do some logic around parsed GraphQL request. Maybe we should generalize it and provide some server execution hooks?

i.e.

• when generating the schema we can provide schema generation hooks
• when processing raw HTTP requests we can use Spring WebFilter (or Ktor equivalent) - great for reading headers (e.g. doing some auth stuff) or adding headers to the response but we shouldn't be parsing the request body (which is our GraphQL request) in the filters
• during GraphQL engine execution we can use instrumentation

What we are missing is the ability to customize server behavior - yes, folks can provide their own server and override the methods but it is pretty cumbersome. Default server logic is as follows

1. parse HTTP request to a GraphQL request
2. generate GraphQL context
3. process GraphQL request

Wondering whether we should update it to (hook names TBD)

1. [new] hook -> willParseRequest
2. parse HTTP request to a GraphQL request
3. [new] hook -> didParseRequest
4. generate GraphQL context
5. [new] hook -> willProcessRequest
6. process GraphQL request
7. [new] hook -> didProcessRequest

Originally posted by @dariuszkuc in #1663 (comment)