Consider generating random salt when creating a delegation

### **Description**

When no `salt` is specified in calls to `createDelegation` and `createOpenDelegation`, the default value `0x00` is used.

Generating a random salt would be safer as a caller creating a delegation would not expect two delegations created with the same arguments to be considered _the same delegation_ which is the current behaviour.

### **Technical Details**

Consider exporting a `generateSalt` function from @metamask/delegation-core so that core consumers don't need to reproduce this function.

### **Acceptance Criteria**

- Are metrics required?
- Are translations required?
- Cases to satisfy
- XYZ should work
- Etc.

Scenario: xxxx
- GIVEN a user is in x state
- WHEN a user does x
- AND a user does x
- THEN x should occur

### **References**

- References go here.
- Issue numbers. Links.
- Slack threads.
- Etc.

Start by locating createDelegation and createOpenDelegation in the TypeScript project, then inspect how their default salt is set. Check the @metamask/delegation-core API for where a generateSalt export would belong. Done should be defined by an agreed random-salt behavior and an explicit set of acceptance scenarios; the current criteria are placeholders.

Written by the indexing model from the issue text.