Refactor smithy_types::Error to be more flexible (Breaking Change)

[rcoh]

Issue #, if available: #422

Description of changes:
S3 (and probably other) services have errors with an additional set of metadata that must be recorded. This diff does a few things:

1. Make the internal fields of smithy_error::Error private (as they should have been from the beginning)
2. Adds a builder to easily construct errors
3. Adds a property bag to enable storing additional fields on the error.

This will make it straightforward to add per-service customizations to record additional error metadata. Services will define their own extension traits to add strongly-typed property bag accessors.

Breaking Change: This is a breaking change because fields on error that were public are now private.
Fix: Change code that accessed fields directly to use accessors:

// before:
err.message

// after:
err.message()

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[jdisanti]

LGTM!