How to specify default `Max Priority Fee` for custom network

Hi, I searched issues and stackexchange but couldn't find a solution, any help would be appreciated!

## Describe the issue
when sending token with a custom network that has custom RPC methods implementation , I can see that Metamask fetches:
- max fee by `eth_gasPrice`
- gas limit by `eth_estimateGas`.
- max priority fee: ??

However, I was not able to figure out how metamask determines the default `Max Priority Fee`, which is currently the same as gas limit.

## Expected behavior
I hope to be able to set a default `Max Priority Fee` value (such as 1.5), either by hardcoding it somewhere, or deriving it from any RPC calls (I can customize any RPC calls for this network).

## Screenshots
from the screenshot we can see that after clicking `send` and `Next`, on the gas settings page, `max priority fee` is the same as `max fee`

## Browser details
- OS: OS X
- Browser: Chrome
- MetaMask Version: 10.8.1

No files or tests are named. Start by tracing the custom-network transaction fee flow and the eth_gasPrice and eth_estimateGas entry points, then identify how max priority fee is currently selected. Done means a custom network can provide or configure a default max priority fee without making it equal to the max fee.

Written by the indexing model from the issue text.