[ENHANCEMENT] Add custom metadata flag to robot.toml

[febb0e]

Is your enhancement request related to a problem? Please describe.
Prefacing the upcoming RF 7.5 feature for custom metadata support. I would like to add the CLI flag to robot.toml.

Describe the solution you'd like
The toml entry should be able to define a list of valid custom metadata entries, that are translated to the --custommetadata flag in the upcoming RF 7.5 version.

Additional context
Link to PR