Add plugin APIs for providing custom HSTS, HPKP, and OCSP databases

• include/wget/wget.h: Add plugin API declarations
• libwget/hsts.c: Make wget_hsts_db_t extensible
• libwget/hpkp.c: Make wget_hpkp_db_t extensible
• libwget/ocsp.c: Make wget_ocsp_db_t extensible
• libwget/plugin.c: Add virtual functions for plugin API
• src/plugin.c: Add plugin API implementation
• src/wget_plugin.h: Likewise
• src/option.c: Add code to call the implementation
• src/wget.c: Likewise
• tests/test-plugin-dummy: Add plugin with dummy databases for testing
• tests/Makefile.am: Add target libplugindb.la
• tests/test-plugin.c: Add tests for custom databases
• unit-tests/test.c: Add tests for new HPKP functions
• libwget/ssl_gnutls.c: Adapt to API changes
• examples/http_multi_get.c: Likewise
• fuzz/libwget_hpkp_fuzzer.c: Likewise
• fuzz/libwget_hsts_fuzzer.c: Likewise
• fuzz/libwget_ocsp_fuzzer.c: Likewise

Custom HSTS, HPKP and OCSP databases and related plugin API have been implemented as per specifications at https://gitlab.com/akash_rawal/wget2/wikis/Stage-4