[ECS] [request]: ECS-AGENT: Increase input buffer size in envfile.go

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**Tell us about your request**
Customers sometimes need to pass in large environment variables from env files in ECS. ECS-Agent uses bufio to read these values from the files. By default, bufio's maximum buffer is 64kb.

I would like to request to increase this maximum, or make it configurable by the customer.

**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?**
I have a customer, for example, whose application uses an 80+kb json object for configuration. The pipeline fails because ECS-Agent throws an error for bufio.Scanner() Token too long. The maximum size for a buffer can be set explicitly in the source code. Currently, ECS-Agent uses the default size.

**Are you currently working around this issue?**
Currently, the only workaround is to use a shortened version of the configuration which comes in under the 64kb maximum size. This is not sustainable long-term for my customers.

**Additional context**
Anything else we should know?

**Attachments**
Bufio docs - https://pkg.go.dev/bufio
envfile.go in ecs-agent (see line 560) - https://github.com/aws/amazon-ecs-agent/blob/master/agent/taskresource/envFiles/envfile.go
StackOverflow mentioning how to configure larger buffer - https://stackoverflow.com/questions/21124327/how-to-read-a-text-file-line-by-line-in-go-when-some-lines-are-long-enough-to-ca

Start with ecs-agent's agent/taskresource/envFiles/envfile.go around line 560 and review how bufio.Scanner reads environment-file values. Compare the current behavior with the bufio documentation and determine whether the requested change should use a larger fixed buffer or customer configuration. Done means values larger than 64 KB no longer fail with a Scanner token-too-long error.

Written by the indexing model from the issue text.