Ability to set `datanames` in module's constructor

[donyunardi]

Summary

Related to insightsengineering/teal#1436.

We need to review all modules to ensure there is a way for users to set datanames in the module. Upon review, we identified two modules that require updates:

• tm_missing_data
• tm_variable_browser

Acceptance Criteria

• Update the design of tm_missing_data and tm_variable_browser to allow datanames to be set in the module's constructor.
• Ensure that every module provides a way to configure datanames (via argument, data extract spec, or other means) in the module's constructor.
• Review and update roxygen note, if necessary

[llrs-roche]

• tm_variable_browser: it has a datasets_selected that could be used for datanames in line with tm_data_table().
• tm_missing_data(): Adding a datasets_selected, seems to be the most consistent approach.

Searching for other potential modules (from the 15 exported by tmg) with less flexibility on datanames:

• tm_front_page(): Currently has an "all" or nothing approach (via show_metadata). Given the documentation and the architecture I guess the tables are meant to be created externally. But perhaps we could enable showing the metadata of only some selected datasets would be useful.
• tm_file_viewer(): Doesn't have the option to set datanames, but I think this is fine, as the module is about files not datasets.