I am creating the web wallet for the bitcoin , I want to allow web wallet users to set the transactions fee .
Is their is any rpc call available for setting the transactions fee (for setting transactions priority) ?
When creating a raw transaction: Inputs - Outputs = Fee. If creating a raw transaction you'll need to calculate this yourself.
The link above will provide examples. You use their built in calculation or build your own raw transaction and set whatever fee you like by using Inputs - Outputs. There is no "rpc call" to set it that i can find or have heard of.
You can set variables for WALLETS in the bitcoincore config, but as you stated, you're working on managing multiple wallets for multiple users. Not sure if having bitcoincore manage every one of your wallets for a large number of users is viable or even possible. If i'm wrong, check this out:
Either way that's not the answer as it's not an "rpc call".