feat: add secp256r1 add/double precompiles

[shuklaayush]

Adds SECP256R1_ADD and SECP256R1_DOUBLE precompiles for secp256r1 (P-256) operations. The implementation is similar to secp256k1 and I reused the WeierstrassAdd and WeierstrassDouble chips

Run the tests:

cargo t -r test_secp256r1_add_simple && cargo t -r test_secp256r1_double_simple && cargo t -r test_secp256r1_mul_simple

Since the constant $a \neq 0$ for this curve, the constraints in the WeierstrassDouble chip were failing on padding rows. I modified it to only check the constraints on rows where the is_real column is set

Addresses #230

[shuklaayush]

@puma314 For full signature verification, I'll have to add a precompile for secp256r1 point decompression (akin to SECP256K1_DECOMPRESS). The SECP256K1_DECOMPRESS precompile assumes that the curve constant $a = 0$ which is not the case for secp256r1.
Should I generalize this precompile for all SW curves (by adding more columns to the air) or should I create a new precompile specifically for secp256r1? I'm thinking of following up with this in a separate PR

[puma314]

Hey @shuklaayush this is great work! We recently refactored a lot of stuff around our precompiles for EC ops to be more general, so I think you should probably merge in the latest main to this PR? I can take a look afterwards!

[shuklaayush]

@puma314 Done! Should be good for review

[puma314]

At some point we should add these precompiles & get them audited, but closing for now since there are a ton of merge conflicts.