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ARROW-9517: [C++/Python] Add support for temporary credentials to S3Options

### Background
AWS provides a mechanism for using temporary credentials to access AWS resources.  When accessing AWS resources with a set of temporary credentials,[ users must provide a session token](https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp_use-resources.html) in addition to the usual access key id and secret access key.

This PR adds support for providing a session token when initializing an S3FileSystem.

Additionally, this PR adds support for auto-refreshing temporary credentials via STS AssumeRole: instead of passing explicit credentials, users supply the arn of a role to assume, and an `STSAssumeRoleCredentialsProvider` will be created to handle fetching temporary credentials by assuming this role.

### Changes
#### C++
- updated `S3Options.FromAccessKey` and `S3Options.ConfigureAccessKey` to accept an optional `session_token` argument (defaulting to empty string, in accordance with the convention of [Aws::Auth::AWSCredentials](https://sdk.amazonaws.com/cpp/api/0.12.9/d4/d27/class_aws_1_1_auth_1_1_a_w_s_credentials.html) as implemented in the AWS C++ SDK.)
- updated `S3FileSystem` implementation to initialize S3Client with CredentialsProvider instead of directly passing AwsCredentials (enabling auto-refreshing of AwsCredentials from provider)
- added `S3Options.GetSessionToken` method
- added `S3Options.FromAssumeRole` and `S3Options.ConfigureAssumeRoleCredentials` to support auto-refreshing temporary credentials via STS AssumeRole.

#### Python
- updated cdef `CS3Options` to reflect updates to C++ library
- added optional `session_token` argument to `S3FileSystem.__init__`.

### Testing
- updated `test_s3_options` python unittest to include a mock session_token when initializing S3FileSystem.
- updated `test_s3_options` python unittest to include relevant scenarios for provided a `role_arn` when initializing S3FileSystem.
- updated `s3fs_test.cc` c++ tests to cover changes to FromAccessKey as well as the newly added FromAssumeRole.
- successfully ran ctest suite and pytest suite with `--enable-s3` option.

Closes #7803 from corleyma/dev/ARROW-9517-add-s3-option-session-token

Authored-by: Matt Corley <mattc@23andme.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>