OGM-3074: Add onContrast style for BPKSwitch

[melwaraki]

https://skyscanner.atlassian.net/browse/OGM-3074

We noticed that the background for BPKSwitch was virtually invisible when used on our dark surface colour in the search controls: https://skyscanner.slack.com/archives/C07R4T86U3Y/p1731427038598569

After a discussion with design (https://skyscanner.slack.com/archives/C07R4T86U3Y/p1731427744624449) we agreed to add a contribution to Backpack to enable this change. This decision was documented here: https://skyscanner.atlassian.net/wiki/spaces/OGM/pages/1243021492/Decision+Log+-+Backpack+Switch+Contributions

Similarly to what we have for BPKSpinner, this PR implements the new changes to add a style for BPKSwitch.

This change is currently only implemented in the Obj C code, but we could add it to Swift UI in this PR or a future ticket. I'd like to get your review first to see if this is the correct approach 😄

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