feat(types): improve streaming payload types

[kuhe]

closes aws/aws-sdk-js-v3#4933

JSv3 codegen diff: aws/aws-sdk-js-v3#4987

This PR replaces manually constructed unions in streaming payloads with an alias type that has attached explanatory documentation.

Old Streaming input/output types:

export type PutObjectCommandInput = Omit<PutObjectRequest, "Body"> & {
  Body?: PutObjectRequest["Body"] | string | Uint8Array | Buffer;
};

export interface GetObjectCommandOutput extends WithSdkStreamMixin<GetObjectOutput, "Body">, MetadataBearer {}

Note the blob type's base type is Readable | Blob | ReadableStream, making the full union of the input:
string | Uint8Array | Buffer | Readable | Blob | ReadableStream.

The problem with these inlined unions is that it isn't clear why the type is so wide, and what types are truly compatible with a particular streaming payload.

New streaming intput/output types:

export interface PutObjectCommandInput extends Omit<PutObjectRequest, "Body"> {
  Body?: StreamingBlobPayloadInputTypes;
}

export interface GetObjectCommandOutput extends Omit<GetObjectOutput, "Body">, MetadataBearer {
  Body?: StreamingBlobPayloadOutputTypes;
}

The type union aliases provide a description of the union contents, why they are typed this way, and when you might want to use each union member.