ease upgrade path for programmatic default values

[yaacovCR]

#3814 added default value validation and coercion, deprecating astFromValue() (which was unsafe, used serialize() to uncoerce input values provided in the internal format) and replacing it with a call to valueToLiteral() which safely operates on external values.

This PR makes that change backwards compatible by reintroducing it as a new config option of externalDefaultValue instead of replacing the existing option of defaultValue.

defaultValueLiteral stays as its open top-level config option, but is subsumed under externalDefaultValue within the GraphQLArgument/GraphQLInputField as it is always external.

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[yaacovCR]

To summarize, cross-posting from #4288 (comment):

In v16, the available default value option on GraphQLArgumentConfig and GraphQLInputField was just defaultValue and that represented the internal value and so external values were never validated no matter how supplied. If the schema was built from SDL, we converted the default value AST into the internal value immediately by way of valueFromAST(), which just erased the default if it was invalid, if it was programmatically supplied, we just used whatever was suppllied.

In v17 currently, prior to the proposed #4296, we adopted @leebyron 's work and replaced that with two options, defaultValue and defaultValueLiteral where the former was the external value programmatically supplied, and the lateral was the ValueNode from the schema built from buildSchema() or extendSchema(), validating both of these within validateSchema(), and so we retain on GraphQLArgument and GraphQLInputField types a single defaultValue property of type DefaultValueUsage which either has a value or a literal property.

After #4296, we have both. On the config, to disambiguate, we use externalDefaultValue for the programmatically supplied option, so defaultValue as previously supplied stays the same. Within the objects, the new DefaultValueUsage object is stored under externalDefaultValue but the old internal default value if supplied will be under defaultValue. The new config options take precedence over the old.

Converting from the old to new and vice a versa

Coercing the external values for the defaults to the input values can be done easily using the coerceInputValue() or the coerceInputLiteral functions, (although it's not necessarily safe until the schema has been validated, in particular default values may contain themselves recursively which would be quite unsafe).

If you want to safely turn an internal value into something external, we have no function to do this in v16 or v17. But if you want, you can do what's done unsafely in v16 for introspection, you can use astFromValue() to move from an internal value to an AST, which is always external, and can now be supplied as the default via the defaultValueLiteral option.