graphql: Treat missing variables as null values

Instead of immediately reporting an error, treat missing variables as nulls
and let the rest of the execution logic deal with nulls

build(deps): bump follow-redirects in /tests/integration-tests

Bumps [follow-redirects](https://github.com/follow-redirects/follow-redirects) from 1.14.4 to 1.14.7.
- [Release notes](https://github.com/follow-redirects/follow-redirects/releases)
- [Commits](follow-redirects/follow-redirects@v1.14.4...v1.14.7)

---
updated-dependencies:
- dependency-name: follow-redirects
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

graph, graphql: Properly validate GraphQL queries before executing them

Uses `graphql-tools-rs` for the actual validation.

Validation can be turned off by setting `DISABLE_GRAPHQL_VALIDATIONS=true` in the environment.

chain head listener: could log be reordered?

chain head listener: add blocking thread pool contention check

chain head listener: more logs

chain head listener: change watcher deadlock detection to avoid spuri…

…ous deadlock detection

chain head listener: change watcher deadlock detection to avoid spuri…

…ous deadlock detection

chain head listener: add number to logging contexts

chain head listener: switch atomic for mutex