chore: use discriminated unions over string enums

Refs: https://rlee.dev/stop-misusing-typescript-string-enums#heading-drawbacks-of-string-enums

Author: trivikr

packages/middleware-flexible-checksums/src/constants.ts

@@ -1,44 +1,48 @@
 /**
  * Determines when a checksum will be calculated for request payloads.
  */
-export enum RequestChecksumCalculation {
+export const RequestChecksumCalculation = {
   /**
    * When set, a checksum will be calculated for all request payloads of operations
    * modeled with the {@link httpChecksum} trait where `requestChecksumRequired` is `true`
    * AND/OR a `requestAlgorithmMember` is modeled.
    * {@link https://smithy.io/2.0/aws/aws-core.html#aws-protocols-httpchecksum-trait httpChecksum}
    */
-  WHEN_SUPPORTED = "WHEN_SUPPORTED",
+  WHEN_SUPPORTED: "WHEN_SUPPORTED",
 
   /**
    * When set, a checksum will only be calculated for request payloads of operations
    * modeled with the {@link httpChecksum} trait where `requestChecksumRequired` is `true`
    * OR where a `requestAlgorithmMember` is modeled and the user sets it.
    * {@link https://smithy.io/2.0/aws/aws-core.html#aws-protocols-httpchecksum-trait httpChecksum}
    */
-  WHEN_REQUIRED = "WHEN_REQUIRED",
-}
+  WHEN_REQUIRED: "WHEN_REQUIRED",
+} as const;
+
+export type RequestChecksumCalculation = (typeof RequestChecksumCalculation)[keyof typeof RequestChecksumCalculation];
 
 /**
  * Determines when checksum validation will be performed on response payloads.
  */
-export enum ResponseChecksumValidation {
+export const ResponseChecksumValidation = {
   /**
    * When set, checksum validation MUST be performed on all response payloads of operations
    * modeled with the {@link httpChecksum} trait where `responseAlgorithms` is modeled,
    * except when no modeled checksum algorithms are supported by an SDK.
    * {@link https://smithy.io/2.0/aws/aws-core.html#aws-protocols-httpchecksum-trait httpChecksum}
    */
-  WHEN_SUPPORTED = "WHEN_SUPPORTED",
+  WHEN_SUPPORTED: "WHEN_SUPPORTED",
 
   /**
    * When set, checksum validation MUST NOT be performed on response payloads of operations UNLESS
    * the SDK supports the modeled checksum algorithms AND the user has set the `requestValidationModeMember` to `ENABLED`.
    * It is currently impossible to model an operation as requiring a response checksum,
    * but this setting leaves the door open for future updates.
    */
-  WHEN_REQUIRED = "WHEN_REQUIRED",
-}
+  WHEN_REQUIRED: "WHEN_REQUIRED",
+} as const;
+
+export type ResponseChecksumValidation = (typeof ResponseChecksumValidation)[keyof typeof ResponseChecksumValidation];
 
 /**
  * Checksum Algorithms supported by the SDK.