feat: AWS_PROFILE env var support

[bilalq]

Description of changes:

Motivation

This CLI tool previously defaulted to using the default AWS profile and allowed the user to override that by passing a --profile option.

However, the AWS CLI and other tools will respect the AWS_PROFILE environment variable as well. This can be observed in documentation here:
https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-files.html#cli-configure-files-using-profiles

To bring consistency with other tools and workflows and convenience for existing users of the environment variable, support for that env variable would be appreciated.

Implementation

Commander natively supports using environment variables as inputs for options. An example can be seen here:
https://github.com/tj/commander.js/blob/master/examples/options-env.js

However, the fluent .option method doesn't support setting that configuration. An instance of the Option class has to be created to set that. The diff required here is fairly small:

-  .option("-p, --profile <profile>", "AWS profile to use", "default")
+  .addOption(new program.Option("-p, --profile <profile>", "AWS profile to use").default("default").env("AWS_PROFILE"))

Impact

This change updates the command definition to support a value from that environment variable as well. The order of precedence is as follows:

1. Use the value explicitly provided via the --profile option.
2. If the option was not passed, use the value of AWS_PROFILE environment variable.
3. If AWS_PROFILE is not set, use "default"

If the user does not have the AWS_PROFILE environment variable set, then there is no user-facing impact or change at all.

After this change, this is what the help output looks like:

$ xray t -h
Usage: xray traces|t [options]

Find and render trace timeline. All parameters are optional.

Options:
  -s, --start <start>               Start time (minutes ago) (default: 5)
  -e, --end <end>                   End time (minutes ago) (default: 0)
  -as, --absolute-start <start>     Start time (ISO 8601)
  -ae, --absolute-end <end>         End time (ISO 8601)
  -f, --filter-expression <filter>  Filter expression. Must be inside double or single quotes ("/')
  -p, --profile <profile>           AWS profile to use (default: "default", env: AWS_PROFILE)
  -h, --help                        display help for command

Checklist

• Update tests
• Update docs

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No test setup exists in the package, so tests were not added.