Move load_file into iomgr

BUILD

Makefile

binding.gyp

build.yaml

config.m4

gRPC.podspec

grpc.gemspec

package.xml

src/core/lib/iomgr/load_file.c

src/core/lib/iomgr/load_file.h

...y/credentials/google_default/google_default_credentials.c

src/core/lib/security/transport/security_connector.c

src/python/grpcio/grpc_core_dependencies.py

test/core/bad_ssl/servers/cert.c

test/core/internal_api_canaries/support.c

test/core/iomgr/load_file_test.c

test/core/security/create_jwt.c

test/core/security/fetch_oauth2.c

test/core/util/one_corpus_entry_fuzzer.c

tools/doxygen/Doxyfile.core.internal