[v2] Expose io_chunksize S3 transfer config

[hssyoo]

This PR exposes an existing S3 transfer config io_chunksize so that it's configurable via the shared config file.

Copied from doc update in the PR:

When a GET request is called for downloads, the response contains a file-like
object that streams data fetched from S3. Chunks are read from the stream and
queued in-memory for writes. io_chunksize configures the maximum size of
elements in the IO queue. This value can be specified using the same semantics
as multipart_threshold, that is either as the number of bytes as an
integer, or using a size suffix.
Increasing this value may result in higher overall throughput by preventing
blocking in cases where large objects are downloaded in environments where
network speed exceeds disk write speed.

[kdaily]

Question: should this get added to the TransferConfig class as well?

aws-cli/awscli/s3transfer/__init__.py

Line 667 in efc1154

class TransferConfig:

I ask because I don't think sending it through kwargs is going to persist it on the object created here:

https://github.com/aws/aws-cli/pull/9678/files#diff-280c24f20aa46551fbfd53f1910ece8ac396fae5de0956cf3555015592d45161R217

[hssyoo]

Question: should this get added to the TransferConfig class as well?

aws-cli/awscli/s3transfer/__init__.py

Line 667 in efc1154

class TransferConfig:

I ask because I don't think sending it through kwargs is going to persist it on the object created here:

https://github.com/aws/aws-cli/pull/9678/files#diff-280c24f20aa46551fbfd53f1910ece8ac396fae5de0956cf3555015592d45161R217

The referenced code is effectively dead. It's only used for a GameLift customization [source]. The TransferConfig object used everywhere else is here:

aws-cli/awscli/s3transfer/manager.py

Line 53 in b9b9e44

class TransferConfig:

Are you actually seeing the value not being persisted when configuring io_chunksize?