Add templated functions for `safe_strto[f|d|i32|i64|u32|u64]` (#14789)

* Add templated functions for `safe_strto[f|d|i32|i64|u32|u64]`

While working on 14330 I noticed that there is no templated functions for
`safe_strto[f|d|i32|i64|u32|u64]`. As a result, an additional wrapper
has to be created in different places to apply the typename in a
templated class or function. Examples include the existing implementations
in `tensorflow/core/kernels/string_to_number_op.cc` (`Convert`),
`tensorflow/core/lib/strings/proto_text_util.h` (`ProtoParseNumeric`),
and in 14330.
It might make sense to add a templated function for `safe_strto[f|d|i32|i64|u32|u64]`
to avoid existing and future code duplications.
This fix adds
```
template<typename T>
bool SafeStringToNumeric(StringPiece s, T* value);
```
to address the above mentioned issue.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Move ProtoParseNumeric to numbers.h, and use overloading in template

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>