Fix build breaks for S3 unit test
Fix build breaks for S3 unit test

This PR tries to address the issue raised in 35040 where
S3 unit test failed due to `gtl::string_as_array`.

This PR change `string_as_array` into getting the pointter of
the first char of the string. By std standard, `std::string`'s
memory is continuous so &s[0] will give the right memory location.

This PR fixes 35040.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

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