setChecksum* related methods for PostPolicy

[qwwqe]

I've noticed there aren't any checksum-related methods defined on PostPolicy, such as something comparable to the SetChecksum receiver implemented in the Go version of the minio SDK. Is this a deliberate design choice, or would a PR request adding such functionality be welcomed?

If the various algorithms were to be broken into distinct methods, the implementation might look something like this:

export class PostPolicy {
  ...

  setChecksumSHA256(checksum: string) {
    if (!checksum) {
      throw new Error('SHA256 checksum cannot be null')
    }

    this.policy.conditions.push(['eq', '$x-amz-checksum-algorithm', 'SHA256']);
    this.formData['x-amz-checksum-algorithm'] = 'SHA256';

    this.policy.conditions.push(['eq', '$x-amz-checksum-sha256', checksum]);
    policy.formData['x-amz-checksum-sha256'] = checksumSha256;
  }

  ...
}

Alternatively, a combined approach might look like this (or using native enum if that's preferred):

const ChecksumAlgorithmHeaders = {
  CRC32: 'x-amz-checksum-crc32',
  CRC32C: ' x-amz-checksum-crc32c',
  SHA1: 'x-amz-checksum-sha1',
  SHA256: 'x-amz-checksum-sha256',
}

export type ChecksumAlgorithm = keyof typeof ChecksumAlgorithmHeaders

export class PostPolicy {
  ...

  setChecksum(algorithm: ChecksumAlgorithm, checksum: string) {
    if (!ChecksumAlgorithmHeaders[algorithm]) {
      throw new Error(`Unknown checksum algorithm: ${algorithm}`) 
    }

    if (!checksum) {
      throw new Error('Checksum cannot be null')
    }

    this.policy.conditions.push(['eq', '$x-amz-checksum-algorithm', algorithm]);
    this.formData['x-amz-checksum-algorithm'] = algorithm;

    this.policy.conditions.push(['eq', `$${ChecksumAlgorithmHeaders[algorithm]}`, checksum]);
    this.formData[ChecksumAlgorithmHeaders[algorithm]] = checksum;
  }

  ...
}

Relevant AWS documentation can be found here and here.