[Firelens] [request]: Override parsers.conf in firelens options

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**Tell us about your request**
Ability to add/mount the parsers.conf file for fluentbit into the image sourced from s3 in the same way we add the fluentbit-extra.conf

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

We’d like to use firelens without building and maintaining an extra container. We’re almost there but we need to change the json parser to accept an iso format for parsing the time stamp.

**Are you currently working around this issue?**
Building a separate image

**Additional context**
Ideally we’d like to be able to do something like this:

```
{
"containerDefinitions":[
{
"essential":true,
"image":"906394416424.dkr.ecr.us-west-2.amazonaws.com/aws-for-fluent-bit:latest",
"name":"log_router",
"firelensConfiguration":{
"type":"fluentbit",
"options":{
"parser-file-type":"s3",
"parser-file-location": "arm:s3:::bucketname/parsers.conf"
}
}
}
]
}
```

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start with the ECS firelensConfiguration options example in the issue and investigate how parser-file-type and parser-file-location are handled for Fluent Bit. Done means FireLens can source an overriding parsers.conf from S3 and use it to support the requested ISO timestamp format without a separately maintained image.

Written by the indexing model from the issue text.