Datasource config, set OTel table names by default

[SpencerTorres]

When creating a new datasource, the otel tables will now be set to otel_logs and otel_traces by default.
I see users get confused by this a lot, now they don't have to worry about setting the otel table names.

This does not affect existing datasources with tables already set. If the value is already set or an empty string, then the defaults will not be applied. Defaults will be applied only when the tables are undefined (such as when a new datasource is created).

Changes:

• added constants for default logs table and trace table
• used those constants everywhere (placeholder text, config value, tests)
• in the datasource config, otel tables will now be set to the default values
• updated changelog

[aangelisc]

LGTM, thank you @SpencerTorres!