[FIRRTL][CAPI] Allow constructing integers larger than 64 bits

[SpriteOvO]

We are building firrtl.constant IRs from Chisel through MLIR, mlirIntegerAttrGet is used to construct constants, however it only accepts a int64_t value as an argument, so constants larger than 64 bits are not possible to be constructed at the moment.

Upstream report: llvm/llvm-project#84190 (comment)

Refer to @mikeurbach's PR #6787, I propose to likewise add a string hack to workaround this problem. Using this new added C-API firrtlAttrGetIntegerFromString, users (e.g. Chisel) could construct a MlirIntegerAttr from a big int string.

[mikeurbach]

I think this is reasonable given the current limitations. Can you please add some tests of positive and negative numbers both larger and smaller than what fits in 64 bits?

[mikeurbach]

Thanks for adding the tests. This looks good to me to get something going.

[SpriteOvO]

Tested working fine in Chisel binder, thanks to Mike's review.