Cherry-pick #18585 to 7.x: Add support for array parsing in azure-eventhub input

[narph]

Cherry-pick of PR #18585 to 7.x branch. Original message:

What does this PR do?

Adds support to parse array of eventhub messages into separate events.
Expected type was an object containing a list of messages.

Why is it important?

List of eventhub messages were published in single Elasticsearch event.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Ex before:

          "message" : """[{"data":"{\"Id\":\"https://testnamekey.vault.azure.net/keys/weewewew/....\",\"VaultName\":\"testnamekey\",\"ObjectType\":\"Key\",\"ObjectName\":\"weewewew\",\"Version\":\"....\",\"NBF\":null,\"EXP\":null}","dataschema":"#1","id":"tt589454itekjter94oi","source":"/subscriptions/.../resourceGroups/obs-infrastructure/providers/Microsoft.KeyVault/vaults/testnamekey","specversion":"1.0","subject":"weewewew","time":"2020-05-15T12:08:09.616451Z","type":"Microsoft.KeyVault.KeyNewVersionCreated"}]""",
          "azure" : {
            "offset" : 2560,
            "sequence_number" : 3,
            "enqueued_time" : "2020-05-15T12:08:11.232Z",
            "eventhub" : "keyvalut",
            "consumer_group" : "test"
          },
          "input" : {
            "type" : "azure-eventhub"
          }
        },

to:

          "message" : """{"data":"{\"Id\":\"https://testnamekey.vault.azure.net/keys/weewewew/....\",\"VaultName\":\"testnamekey\",\"ObjectType\":\"Key\",\"ObjectName\":\"weewewew\",\"Version\":\"tt589454itekjter94oi\",\"NBF\":null,\"EXP\":null}","dataschema":"#1","id":"....","source":"/subscriptions/...../resourceGroups/obs-infrastructure/providers/Microsoft.KeyVault/vaults/testnamekey","specversion":"1.0","subject":"weewewew","time":"2020-05-15T12:08:09.616451Z","type":"Microsoft.KeyVault.KeyNewVersionCreated"}""",
          "azure" : {
            "offset" : 2560,
            "sequence_number" : 3,
            "enqueued_time" : "2020-05-15T12:08:11.232Z",
            "eventhub" : "keyvalut",
            "consumer_group" : "test"
          },
          "input" : {
            "type" : "azure-eventhub"
          }
        },

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

💚 Build Succeeded

Expand to view the summary

Build stats

• Build Cause: [Started by user Mariana Dima, Replayed Fails to start with pcap.OpenLive failed(): : No such device exists #2]

• Start Time: 2020-05-20T09:43:31.587+0000

• Duration: 45 min 11 sec (2711438)

Test stats 🧪

Test	Results
Failed	0
Passed	1683
Skipped	255
Total	1938

[kaiyan-sheng]

LGTM!