DocumentDB auto discovery: configurable connection parameters

**What would you like Teleport to do?** Add configuration options to the database discovery service to append MongoDB URI parameters (like `maxPoolSize` and `heartbeatFrequencyMS`) to auto-discovered DocumentDB databases. According to this mongo go driver doc, the default for `maxPoolSize` is 100, which I believe teleport would inherit. https://www.mongodb.com/docs/drivers/go/current/connect/specify-connection-options/#options. Proposing the following options be made configurable initially: ``` maxPoolSize minPoolSize maxIdleTimeMS serverSelectionTimeoutMS heartbeatFrequencyMS ``` **What problem does this solve?** Teleport's internal MongoDB driver defaults to a `maxPoolSize` of 100 and sends background health check heartbeats every 10 seconds. When auto-discovering multiple clusters, these defaults can saturate the database connection pool limit and cause issues. These driver parameters cannot be tuned for auto-discovered databases. **If a workaround exists, please include it.** Disable AWS auto-discovery for DocumentDB and statically register each database in teleport config, appending the required parameters directly to the URI string.