[v2] Add ability to get values from properties in subsections

[kdaily]

Issue #, if available:

Continuation of #10109.

Description of changes:

This change adds new parameters to the aws configure get command to specify a sub-section for getting the value of a property. These parameters are analogous to the existing --profile parameter. A parameter will be added for each sub-section type and take a value of the subsection name. Following is the generic pattern for the aws configure get command:

aws configure get
  --<sub-section-type> <sub-section-name> \
  <property>

For example, the following command should get the property sso_region from the sso-session sub-section named my-sso-session:

aws configure get --sso-session my-sso-session sso_region 

Following is an example getting a nested property in a sub-section:

aws configure get \
  --<sub-section-type> <sub-section-name> \
  <nested-section>.<property>

The only sub-section types allowed are services and sso-session.

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