Update --profile flag to override default chain's profile rather than replacing default chain

[dannycjones]

Description of change

A previous PR was open for this change but was very stale: #415.

This change means that a profile can be specified that does not configure any credentials, but may configure other things. Mountpoint does not read any configuration from an AWS Profile other than credentials today but may do in the future.

Before this change, the entire credential chain was replaced with one that would supply profile credentials or error - no fallback. This was not in line with AWS CLI or SDKs.

This should fix #389 since the default chain also allows credential process credentials to be used.

Does this change impact existing behavior?

Yes, it fixes the behavior of the --profile <AWS_PROFILE> flag to be in line with AWS_PROFILE environment variable, as well as --profile <AWS_PROFILE> flag of the AWS CLI.

It also fixes bug #389.

Note: It does not validate that the AWS profile exists, like the AWS CLI does. If the profile does not exist, the parameter will be ignored. Mountpoint already behaves this way today when using the AWS_PROFILE environment variable. Mountpoint previously reporting an error was a side effect of replacing the default chain with a profile provider.

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[dannycjones]

Opened #685 to address crate builds issue.

[dannycjones]

Closing this out since:

• The code base has changed a lot since the original change.
• The original issue (--profile does not correctly use credential_process (Appstream) #389) is now resolved as the CRT now correctly includes the credential process in profile providers.

We should still consider replacing the use of the profile credential provider in Mountpoint with the default chain and a profile override, however I would not currently prioritize this due to a) this issue not being reported and b) the possibility that we remove Mountpoint's dependency on mountpoint-s3-client in the foreseeable future.