Set custom metadata correctly for resumable uploads

[abhis3]

Description

• Fixes a bug where resumable uploads were not honoring and setting user provided metadata on file upload.

  • A lot of our existing unit tests use the multipart path in the code, hence why this case wasn't covered.

• Fixes a bug where certain metadata fields weren't present in CloudStorageObjectMetadata, making them revert to a default value and hence returning incorrect metadata on the final PUT requests.

  • GCS supports these metadata fields, see:
    • https://cloud.google.com/storage/docs/metadata#content-disposition
    • https://cloud.google.com/storage/docs/metadata#content-encoding

• Fixes a bug where GCS metadataUpdate cloud functions were triggered in incorrect situations

Part of fixes to #3398

Scenarios Tested

Tested functionality on a sample iOS app, code here: https://github.com/abhis3/Storage-Emulator-Issue/blob/main/Storage%20Emulator%20Issue/StorageManager.swift

Testing against prod, we see the expected behavior so it's definitely an emulator issue.

Prior to the change, we see that the set metadata looked something like this (using the above app):

FIRStorageMetadata 0x600000b5ca00: {
    bucket = "ios-sdk-issues.appspot.com";
    contentDisposition = inline;
    contentEncoding = identity;
    contentType = "";
    generation = 1646779748099;
    md5Hash = "i91E7z6la2nMpxdarYKjrw==";
    metageneration = 1;
    name = "Profiles/mh1MOpqFgaMI9Iq7kTB72QmsFbYH.jpg";
    size = 31769;
    timeCreated = "2022-03-08T14:49:08.099Z";
    updated = "2022-03-08T14:49:08.099Z";
}

After the changes, metadata looks similar to:

FIRStorageMetadata 0x600000b5ca00: {
    bucket = "ios-sdk-issues.appspot.com";
    contentDisposition = initialCommit;
    contentEncoding = identity;
    contentType = "image/jpg";
    generation = 1646779748099;
    md5Hash = "i91E7z6la2nMpxdarYKjrw==";
    metageneration = 1;
    name = "Profiles/mh1MOpqFgaMI9Iq7kTB72QmsFbYH.jpg";
    size = 31769;
    timeCreated = "2022-03-08T14:49:08.099Z";
    updated = "2022-03-08T14:49:08.099Z";
}

Note the changes in the contentDisposition and contentType fields.

[tonyjhuang]

synced offline, do you mind rebasing off of #4262 and pulling the md -> metadata rename to a separate PR?