Bulk Update should support `_source`

[codebrain]

Currently it is not possible to support the following bulk request:

POST _bulk
{ ""update"" : {""_id"" : ""1"", ""_index"" : ""index1"", ""retry_on_conflict"" : 3} }
{ ""doc"" : {""field"" : ""value""} }
{ ""update"" : { ""_id"" : ""0"", ""_index"" : ""index1"", ""retry_on_conflict"" : 3} }
{ ""script"" : { ""source"": ""ctx._source.counter += params.param1"", ""lang"" : ""painless"", ""params"" : {""param1"" : 1}}, ""upsert"" : {""counter"" : 1}}
{ ""update"" : {""_id"" : ""2"", ""_index"" : ""index1"", ""retry_on_conflict"" : 3} }
{ ""doc"" : {""field"" : ""value""}, ""doc_as_upsert"" : true }
{ ""update"" : {""_id"" : ""3"", ""_index"" : ""index1"", ""_source"" : true} }
{ ""doc"" : {""field"" : ""value""} }
{ ""update"" : {""_id"" : ""4"", ""_index"" : ""index1""} }
{ ""doc"" : {""field"" : ""value""}, ""_source"": true}"

As demonstrated on the following documentation page:
https://www.elastic.co/guide/en/elasticsearch/reference/master/docs-bulk.html

The parameter _source: true is not supported.

[codebrain]

Opened draft PR: #4339

[russcam]

Closing as merged into master, 7.x, 7.6 and 6.x (with changes)