feat: adds optional SCTP SACK timer configuration

[gomaja]

Here is a feature for controlling sctp sack timer optionally.

[gomaja]

@wmnsk
The way this PR is written is consistent with the current code.
However, we may support it differently using options pattern instead, which comes closer to your projection in issue (Support functional options pattern #39).

Let me know you thoughts.

[wmnsk]

The Config structure is meant for M3UA. To avoid mixing up the configs for different layers, I suggest defining SCTPConfig structure within Config and gather all SCTP related parameters in it.

[gomaja]

The Config structure is meant for M3UA. To avoid mixing up the configs for different layers, I suggest defining SCTPConfig structure within Config and gather all SCTP related parameters in it.

Makes sense.
I made some changes, and seems can be improved also later with options pattern (mainly in NewConfig function).