CDRIVER-6152 simplify CSFLE examples

[kevinAlbs]

Motivated by review of https://github.com/10gen/docs-mongodb-internal/pull/15868. This PR intends to update and simplify the CSFLE examples:

• Remove need for separate include to contain examples in one file.
• Use scopes to logically separate sections and isolate lifetimes of objects:

bson_t kms_providers;
{
    // Set `local_key` to a 96 byte base64-encoded string.
    const char *local_key =
        "qx/3ydlPRXgUrBvSBWLsllUTaYDcS/pyaVo27qBHkS2AFePjInwhzCmDWHdmCYPmzhO4lRBzeZKFjSafduLL5z5DMvR/"
        "QFfV4zc7btcVmV3QWbDwqZyn6G+Y18ToLHyK";
    char *as_json = bson_strdup_printf(BSON_STR({"local" : {"key" : "%s"}}), local_key);
    init_bson(&kms_providers, &as_json);
    bson_free(as_json);
}

• Simplify BSON construction. E.g. Use BSON_STR for JSON literals (enables removing quotes):

BSON_STR({"unique" : true, "partialFilterExpression" : {"keyAltNames" : {"$exists" : true}}})

• Use a FAIL macro to abort on error to simplify error handling.