Do we need to do this for APPLICATION_CLOSE?

Add `ngtcp2_conn_get_aead_overhead`.

Allows retrieving the aead overhead passed with
`ngtcp2_conn_set_aead_overhead`.

CMake improvements.

- Allow specifying shared or static with BUILD_SHARED_LIBS.
- Add include directories as PUBLIC to library targets.

This enables using ngtcp2 with CMake `add_subdirectory`.

even if it has NGTCP2_RTB_FLAG_CRYPTO_TIMEOUT_RETRANSMITTED is set.