HDDS-15607. Return x-amz-meta-* user-defined object metadata on S3 GetObject

[Gargi-jais11]

What changes were proposed in this pull request?

s3-tests test_object_set_get_metadata_* fail with KeyError:'meta1'.
PutObject stores custom metadata via getCustomMetadataFromHeaders(), and HeadObject returns it via addCustomMetadataHeaders(), but GetObject does not."

• test_object_set_get_metadata_none_to_good
• test_object_set_get_metadata_none_to_empty
• test_object_set_get_metadata_overwrite_to_empty
• test_object_set_get_unicode_metadata

Fix:
AddedaddCustomMetadataHeaders() to the GET response builder.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-15607

How was this patch tested?

Added IT and UT.
Before Fix:

bash-5.1$ aws s3api put-object \
  --bucket metatest \
  --key foo \
  --body /tmp/bar \
  --metadata meta1=mymeta \
  --endpoint-url http://s3g:9878
{
    "ETag": "\"c157a79031e1c40f85931829bc5fc552\""
}
bash-5.1$ aws s3api get-object \
  --bucket metatest \
  --key foo \
  /tmp/out \
  --endpoint-url http://s3g:9878 \
  --query 'Metadata'
                <------------------------- Didn't return the metadata tag
  
bash-5.1$ aws s3api put-object \
  --bucket metatest \
  --key foo2 \
  --body /tmp/bar \
  --metadata meta1= \
  --endpoint-url http://s3g:9878
{
    "ETag": "\"c157a79031e1c40f85931829bc5fc552\""
}
bash-5.1$ aws s3api get-object \
  --bucket metatest \
  --key foo2 \
  /tmp/out2 \
  --endpoint-url http://s3g:9878 \
  --query 'Metadata'
                 <------------------------- Didn't return the metadata tag

After Fix:

bash-5.1$ aws s3api put-object \
  --bucket metatest \
  --key foo \
  --body /tmp/bar \
  --metadata meta1=mymeta \
  --endpoint-url http://s3g:9878/
{
    "ETag": "\"c157a79031e1c40f85931829bc5fc552\""
}
bash-5.1$ aws s3api get-object \
  --bucket metatest \
  --key foo \
  /tmp/out \
  --endpoint-url http://s3g:9878/ \
  --query 'Metadata'
{
    "meta1": "mymeta"
}
bash-5.1$ aws s3api put-object \
  --bucket metatest \
  --key foo2 \
  --body /tmp/bar \
  --metadata meta1= \
  --endpoint-url http://s3g:9878/
{
    "ETag": "\"c157a79031e1c40f85931829bc5fc552\""
}
bash-5.1$ aws s3api get-object \
  --bucket metatest \
  --key foo2 \
  /tmp/out2 \
  --endpoint-url http://s3g:9878/ \
  --query 'Metadata'
{
    "meta1": ""
}