Use the same type for the group id in the `@semaphore-protocol/group` and `@semaphore-protocol/data` packages.

[vplasencia]

Right now, to create a group, the id could be a BigNumberish but to use the group id in any of the functions in the SemaphoreSubgraph or SemaphoreEthers classes in the @semaphore-protocol/data package, the id should be a string.

[hannahredler]

Hey! I'm a former student of the PSE Core Program. If this is available I'd happily take a look at it :) @vplasencia @cedoor