Separate out common header compression logic to hdrs directory

[maskit]

This is a part of preparation to merge quit-latest branch. Because some of header compression logic are common between HPACK and QPACK, I separated out the logic to hdrs directory.

The code come from quic-latest branch, and it has been working for long time on a QUIC test sever (both H2 and H3 are enabled).

Only change I made is adding n parameter to encode_string and decode_string. On HPACK n is always 7 but QPACK passes other values.

Current:

int64_t encode_string(uint8_t *buf_start, const uint8_t *buf_end, const char *value, size_t value_len);		
int64_t decode_string(Arena &arena, char **str, uint32_t &str_length, const uint8_t *buf_start, const uint8_t *buf_end);

New:

int64_t xpack_encode_string(uint8_t *buf_start, const uint8_t *buf_end, const char *value, uint64_t value_len, uint8_t n = 7);
int64_t xpack_decode_string(Arena &arena, char **str, uint64_t &str_length, const uint8_t *buf_start, const uint8_t *buf_end, uint8_t n = 7);

[maskit]

This is for H2 and H3, but I want an approval from somebody not working on QUIC because this change comes from QUIC branch.

[maskit]

[approve ci autest]