feat: add alias api with previousId support

[1abhishekpandey]

Description

• Added an Alias API with previousId support. Now, previousId can be passed and will be preferred over the existing userId or anonymousId.
• New Alias API:

- (void) alias:(NSString *)newId previousId:(NSString * _Nullable)previousId options:(RSOption * _Nullable) options;

• This is to bring it in parity with the JS SDK so that previousId can be explicitly passed if needed.

NOTE: The change is backwards compatible, as we are introducing a new Alias API which adds a new behaviour to the existing payload schema.

How to test

Objective: We need to test, if previousId is passed then it should be used, else the existing behaviour should continue i.e., userId or anonymousId should be used.

• Make an alias event and explicitly pass previousId field -> It should be set as the previousId.
• Existing API: Make an Identify event, then RESET call and then an alias event (with previousId) -> previousId should be set as passed previousId.
• Existing API: Make an alias event with only newId -> previousId should be set as anonymousId.
• Existing API: Make an alias event with only newId and options -> previousId should be set as anonymousId.
• Existing API: Make an identify event and then an alias event (without previousId) -> previousId should be set as previous userId.
• Existing API: Make an Identify event, then RESET call and then an alias event (without previousId) -> previousId should be set as anonymousId.

[SKannaniOS]

To simplify multiple if conditions, use the Ternary Conditional Operator (?:) as shown below.

NSObject *prevId = previousId ?: [traits objectForKey:@"userId"] ?: [traits objectForKey:@"id"] ?: self.anonymousId;

The base branch was changed.