Allow a max statement timeout for Postgres

### Is your proposal related to a problem?

I'm not able to put an upper bound on postgres queries, which opens us up to bad queries taking down the databas

### Describe the solution you'd like

Add an env var or metadata to the end of HASURA_GRAPHQL_PG_STATEMENT_TIMEOUT

### Describe alternatives you've considered

Looks like the only timeout configuration is in metadata and only terminates idle connections. It appears you have a solution in Hasura Cloud as well but not for OSS

### If the feature is approved, would you be willing to submit a PR?

Maybe

No files or tests are identified in the issue. Start by tracing the existing HASURA_GRAPHQL_PG_STATEMENT_TIMEOUT configuration and metadata handling, then determine how an OSS maximum should be represented and enforced; done means PostgreSQL queries cannot exceed the configured upper bound and the behavior is covered by tests.

Written by the indexing model from the issue text.