Update IDL serializer to write metadata to a separate file

[hpmellema]

Description

Updates the SmithyIdlModelSerializer to write all metadata to a separate metadata.smithy file rather than duplicating the metadata within each generated file.

Generating a separate smithy metadata files avoids potentially duplicating array data.

Detail on issue

For example the following files

/// hello.smithy
$version: "2"

metadata hello = [1]
 
namespace test
 
structure Hello{}

and

/// world.smithy
$version: "2"

metadata hello = [2]
 
namespace test
 
structure World{}

Were previously re-written as:

/// hello.smithy
$version: "2"

metadata hello = [1, 2]
 
namespace test
 
structure Hello{}

and

/// world.smithy
$version: "2"

metadata hello = [1, 2]
 
namespace test
 
structure World{}

This created a problem where, when using these models in a new model, the resulting metadata for hello
would be an array containing [1,2,1,2] effectively duplicating all array data N times (N being number of smithy files generated).

Updated behavior

With this change the following files would now be written for the above example:

/// hello.smithy
$version: "2"
 
namespace test
 
structure Hello{}

/// world.smithy
$version: "2"
 
namespace test
 
structure World{}

/// metadata.smithy
$version: "2"

metadata hello = [1, 2]

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