Add a background colour prop to psammead-section-label

[Bopchy]

Is your feature request related to a problem? Please describe.
Currently, the psammead-section-label component has a background colour of C_GHOST on the Heading. This presents a problem for front page sections that do not have C_GHOST as a background colour (an example of this is the RadioSchedules, which will have a background colour of C_LUNAR as specified in bbc/simorgh#5745)

Describe the solution you'd like
Add a background colour prop to psammead-section-label that defaults to C_GHOST to make it possible to change the section label's background colour as needed

Describe alternatives you've considered
N/A

Testing notes
[Tester to complete]

Dev insight: Will there be any potential regression? etc

• This feature is expected to need manual testing.

Additional context
Add any other context or screenshots about the feature request here.