Require immutable flag on entities #1994
                
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This PR adds a new validation for GraphQL schemas that requires all entities to be explicitly marked as immutable or mutable. This is the first step in transitioning subgraph developers to using immutable entities by default.
As a result, the codegen command will fail if there are any entities without an immutable flag. The error message also provides a hint on what to update and how.
In addition, if GraphQL schemas do not use immutable entities (all entities are mutable), the codegen command will print a warning suggesting that immutable entities should be considered and a link to best practices.
Finally, all example subgraphs have been updated with explicit immutable flags.
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