Remove enum usage for header/trailer in smithy-http and aws-sdk-signers

[ubaskota]

Issue #, if available:

Description of changes:
FieldPosition was an enum, which meant any library implementing the Field protocol had to import smithy-http just to reference the enum values. Switch to Literal["header", "trailer"] so that implementations can pass equality checks using plain strings without depending on smithy-http. Also change identity checks (is) to equality (==) to ensure correctness with string literals.

Testing

• Updated the tests to reflect the changes and verified that all tests pass.

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