Add triplet variables to control abi entries for binarycache

[Osyotr]

VCPKG_IGNORED_ABI_KEYS: list of entries to remove from from vcpkg_abi_info.txt (key1;key2;...;keyN)
VCPKG_ADDITIONAL_ABI_ENTRIES: list of additional entries to add to vcpkg_abi_info.txt (key1 value1;key2 value2;...;keyN valueN)

It's now possible to completely override triplet hash or remove dependencies hashes from abi entries.
⚠Of course, this must be used with caution. ⚠

set(VCPKG_TARGET_ARCHITECTURE x64)
set(VCPKG_CRT_LINKAGE dynamic)
set(VCPKG_LIBRARY_LINKAGE dynamic)

# ⚠ This might break things, but we take the risk
set(VCPKG_ADDITIONAL_ABI_ENTRIES "my_custom_triplet_abi 9C6553BC-E762-458E-BDEF-2FEEBD6388BD")
set(VCPKG_IGNORED_ABI_KEYS "triplet_abi" "powershell")

if("${PORT}" STREQUAL "libpng")
    list(APPEND VCPKG_IGNORED_ABI_KEYS "zlib")
endif()