Add support for operation and service schemas

[haydenbaker]

Description of changes:

• Adds support for generating operation and service schemas:

GET_FORECAST = Schema(
    id=ShapeID("example.weather#GetForecast"),
    shape_type=ShapeType.OPERATION,
    traits=[
        ...
        Trait.new(id=ShapeID("smithy.api#readonly")),
    ],
)
...
WEATHER = Schema(
    id=ShapeID("example.weather#Weather"),
    shape_type=ShapeType.SERVICE,
    traits=[
        ...
        Trait.new(id=ShapeID("aws.protocols#restJson1")),
    ],
)

• Fixes a couple of extraneous issues as well
  • Incorrect dep in example package
  • Bad import when code generating a client for a model that doesn't use the httpHeader trait
    • serialize.py:8:25 - error: Import "Field" is not accessed (reportUnusedImport)

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addressed