DHCP: Add an upper bound to the renew/rebind timeout in RetryConfig

[JarredAllen]

Description

This lets the user set an upper bound on the duration between times smoltcp sends a renew or rebind by setting a max_renew_timeout field to DHCP RetryConfig struct, which defaults to not imposing a cap unless the user explicitly sets one.

Design questions

Use of this feature doesn't follow the official DHCP spec, but the RetryConfig struct already has a min_renew_timeout field which breaks the same spec by allowing the user to change the lower-bound imposed, so I think this is reasonable to add. Feel free to close this PR if you disagree.

I also added #[non_exhaustive] to the RetryConfig struct because this addition will necessarily be backwards incompatible and ineligible for inclusion until v0.11.0, but marking it as #[non_exhaustive] will allow anyone else in the future who wants more configuration options to add them without breaking backwards-compatibility. I can remove this attribute and let any future additions also require a major change instead.

I also didn't put any checks for the user putting max_renew_duration smaller than the existing min_renew_duration. If they do, then they'll silently get max_renew_duration as the chosen value every time. I think this is a reasonable response to an unlikely scenario, but I can do something else instead if you prefer.

Validation

The existing unit tests cover logic in the case where no upper bound is set. I added a new unit test that covers the user setting a custom upper- and lower- bound to ensure that this behavior acts correctly.

I also have a firmware setup which uses this DHCP client that I switched to using this branch and set the maximum, and observed it capping at the maximum like expected.

[codecov]

Codecov Report

Merging #835 (3755e86) into main (01e1acb) will increase coverage by 0.07%.
Report is 3 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #835      +/-   ##
==========================================
+ Coverage   79.52%   79.59%   +0.07%     
==========================================
  Files          78       78              
  Lines       27762    27816      +54     
==========================================
+ Hits        22078    22141      +63     
+ Misses       5684     5675       -9     

Files Changed	Coverage Δ
src/time.rs	86.41% <ø> (ø)
src/socket/dhcpv4.rs	86.32% <100.00%> (+0.71%)	⬆️

... and 1 file with indirect coverage changes

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[Dirbaio]

thank you!