Add API to convert hex string to bytes without using internal blob structure

[ttjsu-aws]

Problem:

Currently within s2n, we have the following API to convert a hex-encoded string to bytes:

int s2n_hex_string_to_bytes(const uint8_t *str, struct s2n_blob *blob);

https://github.com/aws/s2n-tls/blob/main/utils/s2n_blob.h#L47

However this API relies on internal s2n_blob structure and cannot be used within bin/s2nd.c and bin/s2nc.c.

Solution:

Add s2n APIs to convert hex string to bytes without using internal blob structure.

S2N_API int s2n_hex_str_to_bytes_length(const char *str, uint32_t *bytes_len);
S2N_API int s2n_hex_str_to_bytes(const char *str, uint8_t *bytes, uint32_t *bytes_len);

[zaherd]

Duplicate of #2759