Configurable ICE timeout values

[thomaseizinger]

Currently, these are constants:

str0m/src/io/stun.rs

Lines 11 to 14 in 2326934

	pub const STUN_INITIAL_RTO_MILLIS: u64 = 250;
	pub const STUN_MAX_RETRANS: usize = 9;
	pub const STUN_MAX_RTO_MILLIS: u64 = 3000;
	pub const STUN_TIMEOUT: Duration = Duration::from_millis(18_750); // See test for how this is calculated.

Could we make these configurable at runtime via the IceAgent? Our fork currently adjusts these which makes it a bit tedious to keep rebasing :)

[algesten]

Of course. Make setters!