[source-dynamodb] Change 'reserved_attribute_names' from Secret to String in DynamoDB source

### Topic

Change property from a Secret to a String to allow visibility during connection maintenance

### Relevant information

Currently, the DynamoDB connector includes a property called `reserved_attribute_names`. This field allows users to list attributes that might collide with DynamoDB reserved keywords. (introduced in https://github.com/airbytehq/airbyte/pull/20172 )

However, [this property is currently defined as a Secret](https://github.com/airbytehq/airbyte/blob/master/airbyte-integrations/connectors/source-dynamodb/src/main/resources/spec.json#L111). This creates a significant friction point during maintenance: whenever an existing connection needs to be updated (e.g., adding or removing a single attribute name), the previous values are masked and cannot be viewed.

Since this field only contains technical attribute names and no sensitive information, it does not require the security overhead of a secret field.

## Proposed Change

Change the data type of the `reserved_attribute_names` property from a Secret to a standard String.

## Motivation

- Improved UX: Users will be able to see existing reserved names when editing a connection.
- Easier Maintenance: Eliminates the need to re-type the entire list of attributes every time a single change is required.
- Alignment with Data Sensitivity: This field does not hold credentials or keys.

---
**Internal Tracking:** https://github.com/airbytehq/oncall/issues/12066