Fixed checking for external type property on symbol

[timocov]

What behavior changes?

The PR fixes a bug when custom integration's symbol provider set SymbolProperties.EXTERNAL_TYPE to false on a symbol but the codegen still ignores the symbol/shape from generating the code.

Why is this change needed?

Creating a codegen integration allows to override symbol provider. If a symbol provider sets false for SymbolProperties.EXTERNAL_TYPE the codegen skips generating code for this shape completely. This is due to a wrong property check is some places.

How was this validated?

Run tests, built locally and validated the output with the integration.

What should reviewers focus on?

Not much, its a 3 lines PR. But changes in this PR align with other changes in the repo already, e.g.:

• smithy-java/codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/generators/SchemaIndexGenerator.java

  Line 196 in 964c6d3

  if (symbol.getProperty(SymbolProperties.EXTERNAL_TYPE).orElse(false)) {

• smithy-java/codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/generators/SchemaIndexGenerator.java

  Line 75 in 964c6d3

  if (!symbol.getProperty(SymbolProperties.EXTERNAL_TYPE).orElse(false)) {

• smithy-java/codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/generators/SchemaFieldOrder.java

  Line 78 in 964c6d3

  symbolProvider.toSymbol(shape).getProperty(SymbolProperties.EXTERNAL_TYPE).orElse(false);

Additional Links

(this PR is somewhat related to #1198, but isn't really)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.