The IdentityPool credential source. Dictates the retrieval method of the external credential,
which can either be through a metadata server or a local file.
If this is a file based 3P credential, the credentials file can be retrieved using the
file key.
If this is URL-based 3p credential, the metadata server URL can be retrieved using the url
key.
The third party credential can be provided in different formats, such as text or JSON. The
format can be specified using the format header, which returns a map with keys type and
subject_token_field_name. If the type is json, the subject_token_field_name must be
provided. If no format is provided, we expect the token to be in the raw text format.
Optional headers can be present, and should be keyed by headers.