[cmake] Add an option to build the secp256k1 library as shared

Summary: This does not affect the other libraries and make it easier to build the shared libsecp256k1 from the monorepo.

Test Plan:

cmake -GNinja .. -DSECP256K1_BUILD_SHARED=ON
ninja secp256k1

Check the shared lib is produced.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16700