Expose stunGatherTimeout in Agent struct

[stephanrotolante]

Description

Allows to configure STUN timeout from the config

Reference issue

Resolves https://github.com/pion/webrtc/issues/2668

[Sean-Der]

Nice! This looks perfect to me @stephanrotolante

When all the tests go green go ahead and merge!

Then in https://github.com/pion/ice/releases/new create v3.0.4 and do Generate release notes. Then you can open up a pion/webrtc PR and add it to the SettingEngine.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 78.65%. Comparing base (a12f670) to head (d47c426).

Files	Patch %	Lines
agent_config.go	40.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #668 +/- ## ========================================== - Coverage 78.69% 78.65% -0.05% ========================================== Files 42 42 Lines 4698 4703 +5 ========================================== + Hits 3697 3699 +2 - Misses 769 771 +2 - Partials 232 233 +1 ``` | [Flag](https://app.codecov.io/gh/pion/ice/pull/668/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [go](https://app.codecov.io/gh/pion/ice/pull/668/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `78.65% <57.14%> (-0.05%)` | :arrow_down: | | [wasm](https://app.codecov.io/gh/pion/ice/pull/668/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `22.45% <0.00%> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#carryforward-flags-in-the-pull-request-comment) to find out more.

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