include source stack in aggregated errors in DefaultAzureCredential

As indicated in #1543, error details are swallowed when building an aggregate error message.

Currently, we format each of the errors using to_string, which results in the following:

Error {
    context: Full(
        Custom {
            kind: Credential,
            error: Error {
                context: Message {
                    kind: Credential,
                    message: "Multiple errors were encountered while attempting to authenticate:\nenvironment credential\nIMDS timeout\naz-cli",
                },
            },
        },
        "failed to get bearer token",
    ),
}

This doesn't help the user understand how to fix the issue.

With this update, we recurse through the error sources, building a more detailed message. This results in:

Error {
    context: Full(
        Custom {
            kind: Credential,
            error: Error {
                context: Message {
                    kind: Credential,
                    message: "Multiple errors were encountered while attempting to authenticate:\nenvironment credential - request token error - Server returned error response\nIMDS timeout - operation timed out\naz-cli - 'az account get-access-token' command failed: ERROR: AADSTS70011: The provided request must include a 'scope' input parameter. The provided value for the input parameter 'scope' is not valid. The scope https://storage.azure.com/ offline_access openid profile is not valid. The scope format is invalid. Scope must be in a valid URI form <https://example/scope> or a valid Guid <guid/scope>.  Trace ID: 346f391a-48f2-4e96-849f-9ecd6c589d02 Correlation ID: 7888c325-56ff-4100-8ce2-c8cf41561b40 Timestamp: 2024-01-05 01:31:04Z\nInteractive authentication is needed. Please run:\naz login --scope https://storage.azure.com/\n",
                },
            },
        },
        "failed to get bearer token",
    ),
}

When printed, this message looks like:

Multiple errors were encountered while attempting to authenticate:
environment credential - request token error - Server returned error response
IMDS timeout - operation timed out
az-cli - 'az account get-access-token' command failed: ERROR: AADSTS70011: The provided request must include a 'scope' input parameter. The provided value for the input parameter 'scope' is not valid. The scope https://storage.azure.com/ offline_access openid profile is not valid. The scope format is invalid. Scope must be in a valid URI form <https://example/scope> or a valid Guid <guid/scope>.  Trace ID: 02aecb08-69b4-4a5d-9ebb-784ea788c102 Correlation ID: dddc061a-ca11-4b21-b857-6d765a564597 Timestamp: 2024-01-05 01:43:45Z
Interactive authentication is needed. Please run:
az login --scope https://storage.azure.com/

Azure / azure-sdk-for-rust

include source stack in aggregated errors in DefaultAzureCredential #1546