Add getter and setter for MEV commission

[buffalu]

Problem

Changing the MEV commission requires restating your validator.

Proposed Solution

Add a getter and setter which allows one to get and set mev commission for the validator.

[martinsander00]

Hi @buffalu. I talked with Evan about working on this. I do have a few questions before I make a PR(I'm new to SOL development so hopefully these don't sound dumb to you):

1. Where should the setter be called from? (RPC endpoint, admin CLI, etc)
2. When should the change take effect? (Next epoch vs immediately)
3. Should there be validation? (Max commission limits, change frequency limits?)
4. Persistence? (I assume new value set by the setter should survive restarts but it could also reset to CLI arg value?)

[buffalu]

Hi @buffalu. I talked with Evan about working on this. I do have a few questions before I make a PR(I'm new to SOL development so hopefully these don't sound dumb to you):

1. Where should the setter be called from? (RPC endpoint, admin CLI, etc)
2. When should the change take effect? (Next epoch vs immediately)
3. Should there be validation? (Max commission limits, change frequency limits?)
4. Persistence? (I assume new value set by the setter should survive restarts but it could also reset to CLI arg value?)

1. do it from the admin interface.
2. next epoch
3. yes, make sure not more than 100%
4. doesn't need persistence