resource: make feasibility service registration optional

[grondo]

Problem: There are many cases in the testsuite where the Fluxion resource module is loaded alongside sched-simple for testing, but this currently fails because these two modules now conflict due to the feasibility service.

Since feasibilty is an optional service, and may soon move to a separate module anyway, make feasibility service registration optional.

[grondo]

Thanks! Setting MWP.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 75.4%. Comparing base (ca046b4) to head (d46d09d). Report is 74 commits behind head on master.

Files with missing lines	Patch %	Lines
resource/modules/resource_match.cpp	50.0%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1281 +/- ## ====================================== Coverage 75.4% 75.4% ====================================== Files 111 111 Lines 15361 15360 -1 ====================================== Hits 11588 11588 + Misses 3773 3772 -1 ``` | [Files with missing lines](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1281?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework) | Coverage Δ | | |---|---|---| | [resource/modules/resource\_match.cpp](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1281?src=pr&el=tree&filepath=resource%2Fmodules%2Fresource_match.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework#diff-cmVzb3VyY2UvbW9kdWxlcy9yZXNvdXJjZV9tYXRjaC5jcHA=) | `68.8% <50.0%> (+<0.1%)` | :arrow_up: |

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