[POC][TF FE] Support SavedModel format (with compression) (#16317)

* Added Saved Model proto descriptors

* Included Google's protobuf repository

* Added wstring version of ov::util::directory_exists

* Added initial implementation of Saved Model iterator

# Conflicts:
#	src/frontends/tensorflow/src/frontend.cpp

* Added missing proto files to repository

* Implemented reading of variables index and data files

# Conflicts:
#	src/frontends/tensorflow/src/frontend.cpp

* Renamed class

# Conflicts:
#	src/frontends/tensorflow/src/frontend.cpp

* Fix for cross-platform directory_exists

* Fixed codestyle and simplified code

* CI fixes

* Separeted Saved Model iterator from Proto iterator

* Moved variables index into separate class

* Added initial implementation of reading a variables from
saved model

# Conflicts:
#	src/frontends/tensorflow/src/frontend.cpp

* Added external variable mapping

* Code cleanup

* Commit is for discussion purposes!!!
Implemented RestoreV2 with a workaround for strings
Not optimized, includes mem leak

* In progress...

* Added DT_STRING coverage into decoder_proto

* m_variables_index moved into underlying class

* Updated copyrgihts, added space between license and code

* Moved string constant to separate class

* Added AssignVariableOp operation

* Changed behavior of RestoreV2
Updated stubs for other ops

* Second working implementation, enabled:
Program-only models
Variables reading from data files

* Extended docs

* Fixed dynamic type

* Fixed naming

* Added Snappy submodule to support compression in TF FE

* Enabled Snappy Compression for TF FE

* Make static linkage of Snappy
Changing Warning as error behavior for 3rd party

* CI fixes

* Added Snappy copyright info

* Aligned behavior of StringConstant with UnsupportedConstant

* Added correct naming and removing unused inputs/outputs

.gitmodules

@@ -66,3 +66,6 @@
 [submodule "thirdparty/flatbuffers/flatbuffers"]
 	path = thirdparty/flatbuffers/flatbuffers
 	url = https://github.com/google/flatbuffers.git
+[submodule "thirdparty/snappy"]
+	path = thirdparty/snappy
+	url = https://github.com/google/snappy.git