Fix for optional dependencies of SymbolDependencies

[alextwoods]

Description of changes:
Addresses comment from: #409 (comment)

The issue is that optional dependencies are stored as typed properties on the symbol dependencies and those properties are not merged by smithy's gatherDependencies method. We're adding the smithy_http dependency hundreds of times through out the code, but only once with the optionalDependencies specified.

This PR sets awscrt and aiohttp as optional dependencies on the smithy_http SymbolDependency (undooing changes from #409) and uses a custom gatherDependencies with a custom merge function that merges the OPTIONAL_DEPENDENCIES property.

Resulting dependencies for generated client:

dependencies = [
    "aws_event_stream==0.0.1",
    "smithy_core==0.0.1",
    "smithy_event_stream==0.0.1",
    "smithy_http[awscrt]==0.0.1",
    "smithy_json==0.0.1"
]

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