Revisit GatewayParameters merge logic

[jenshu]

Do you have a suggestion for code improvement or tracking existing technical debt? Please describe.

Currently we get the GatewayParameters for a Gateway by doing a deep merge of the default gwp with the Gateway-specific gwp. This includes merging maps by combining all entries in src/dst, and merging slices by appending src and dst. When a slice/map field is set in both the default and Gateway-specific gwp, there is no way for the Gateway-specific one to completely override the default one.

Describe the solution you'd like

A few possible options are:

• provide a flag that the user can set which determines which merge strategy to use (e.g. deep/shallow merge). We already do something like this in our helm chart. We would need to decide whether this can be set globally or per-Gateway or per-field (which might get messy)
• default to overwriting instead of merging - i think this is safer/more flexible than the current behavior, because if you still wanted to carry over values from default gwp you could just copy over those values into the gw-specific gwp field
• custom merge logic for each specific field in gwp (i.e. do either deep or shallow merge on a case-by-case basis) - this would be hard to maintain, and users may want different merge behavior than what we've implemented

Additional Context

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