Profile mutation and serialization

[jonathanmorley]

Motivation and Context

Supports mutating ProfileSets, so that applications that fetch STS credentials can update a ProfileSet with the new credentials.

Supports serializing ProfileSets, so that those credentials can be written back to disk.

Description

Testing

New test added to src/profile/parser.rs

Checklist

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[rcoh]

I think ultimately, this isn't going to be the right approach. A full round trip of the file is probably not going to be what users want—I think we will almost certainly want to append to the file instead. That way we can preserve comments, keep the users profile in order, and not worry about unsupported features.

For that reason, I think the thing I'd add serializability for is a profile itself. That also removes the need to make ProfileSet mutable.

When parsing profiles, we are able to keep track of line number information. If we include that, you could even update your profile by tracking the offsets.

I don't think we'll be able to merge this PR as is unfortunately for the reasons above.

[rcoh]

let's add docs that clarify that this will replace another profile with the same name. Might as well change the API to return the removed profile if it existed as well

[rcoh]

I think we'll need a few caveats in this method:

1. It is going to delete any comments the user may have had
2. We need to test how it handles multi-line values, e.g. for S3:

[profile development]
s3 =
  max_concurrent_requests = 20
  max_queue_size = 10000
  multipart_threshold = 64MB
  multipart_chunksize = 16MB
  max_bandwidth = 50MB/s
  use_accelerate_endpoint = true
  addressing_style = path

[rcoh]

closing this for now—let us know (or submit a PR) if you'd like to make some more public methods