Merge pull request #6436 from jboeuf/credentials_refactoring

BUILD

Makefile

binding.gyp

build.yaml

config.m4

gRPC.podspec

grpc.gemspec

package.xml

...xt/transport/chttp2/client/secure/secure_channel_create.c

...ext/transport/chttp2/server/secure/server_secure_chttp2.c

src/core/lib/http/httpcli_security_connector.c

src/core/lib/security/context/security_context.c

src/core/lib/security/context/security_context.h

src/core/lib/security/credentials.c

...ib/security/credentials/composite/composite_credentials.c

...ib/security/credentials/composite/composite_credentials.h

src/core/lib/security/credentials/credentials.c

src/core/lib/security/credentials/credentials.h

src/core/lib/security/credentials/credentials_metadata.c

src/core/lib/security/credentials/fake/fake_credentials.c