Remove 3rd-party types from public APIs where possible

[Velfi]

There are 3rd-party types in several public APIs. Where possible

• Newtype those types in a way that hides the inner type
• Remove APIs that don't provide needed functionality
• Replace types with our own

Full list of types to fix

• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/rust-runtime/aws-config/external-types.toml#L35
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/rust-runtime/aws-http/external-types.toml#L9
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/rust-runtime/aws-inlineable/external-types.toml#L18
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/rust-runtime/aws-sig-auth/external-types.toml#L9
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/rust-runtime/aws-sigv4/external-types.toml#L9
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/sdk/sdk-external-types.toml#L20
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/sdk/sdk-external-types.toml#L23
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/aws/sdk/sdk-external-types.toml#L26
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-async/external-types.toml#L2
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-async/external-types.toml#L5
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-client/external-types.toml#L13
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-client/external-types.toml#L18
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-client/external-types.toml#L24
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-client/external-types.toml#L29
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-client/external-types.toml#L32
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-client/external-types.toml#L37
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-eventstream/external-types.toml#L7
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http-tower/external-types.toml#L10
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http-tower/external-types.toml#L7
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http/external-types.toml#L20
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http/external-types.toml#L23
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http/external-types.toml#L26
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http/external-types.toml#L32
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http/external-types.toml#L35
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-http/external-types.toml#L38
• https://github.com/awslabs/smithy-rs/blob/0da92a4319ceef8eae87af09a37e168f6f4adc93/rust-runtime/aws-smithy-xml/external-types.toml#L2

Some of the types listed above may cease to exist as a result of future code reorganization/refactoring. We can park them for now. The ones currently actionable are tracked with the following issues:

• Update cargo-check-external-types to expose external types in public APIs under cargo features #2412
• Remove 3rd-party types from public APIs #2413

List can be updated with:

rg -F 'TODO(https://github.com/awslabs/smithy-rs/issues/1193)' --json . | \
  jq -r 'select(.type == "match") | (.data.path.text) + "#L" + (.data.line_number | tostring)' | \
  sed 's/^\.\///g' | \
  awk -v prefix="- [ ] https://github.com/awslabs/smithy-rs/blob/$(git rev-parse HEAD)/" '{print prefix $0}' | \
  sort

[jdisanti]

primitive::Encoder buffer types fixed in #1209